Describe the PR

This PR is based on the changes of #3674.

Drag and drop files bypasses the file input altogether, which makes native browser validation constraints (i.e. required) not work, as the input does not reflect the dropped files.

Chrome, Firefox, Safari, Edge support dropping files directly on a file input, but IE11 and some older browser versions do not, so we can't take advantage of the native file input drop features (which would simplify the code drastically, and retain proper validity state).

All modern browsers support setting InputElement.files to a FileList array (which will satisfy required constraint), but again IE 11 doesn't support this (could be placed in a try/catch to trap/mask errors).

What's new and changes:

• Drag and drop now attempts to set the files array on the native file input (flattened FileList, filtered by accept criteria), so that required prop works with drag/drop (except on IE, the attempt to set the files is wrapped in a try/catch to suppress the error)
• Drag/drop now respects the accept criteria. Converts the accept prop to an array of regular expressions for testing the file type/extension of dropped files
• Drag and drop did now respects the directory setting (Except IE, which doesn't support directory mode)
• Uses a negative z-index on the opaque file input so that it doesn't interfere with drag/drop on IE (and other browsers). Clicking the custom input label will still open the native file browser (as the label is associated with the input via the for attribute).
• Filename array and files array that is passed to scoped slot are now always flattened (even in directory mode), but includes a new scoped property which provides the files in traversed mode
• Also adds a few fixes to the <b-form-file> docs

TODO:

• accept file matching tests
• Get drag/drop to work correctly on IE 11
• Set dropEffect to 'none' when file(s) can't be dropped (works on all but IE 11, as files are not known until the drop event happens)
• Add optional prop/slot for prompt when file's can't be dropped - or make drop-placeholder slot scoped with a valid scope prop (not applicable to IE 11, as files aren't known until dropped)
• Add notes in docs about limitations of required prop with drag/drop on IE11
• Document requirement of Promise support for drag and drop mode
• Add slots for placeholder prompt, and drop-placeholder prompt, as an alternative to the props.
• Document new slots
• add new slots to package.json meta
• Minor custom SCSS updates to handle overflowing selected file name(s)
• Additional test suites

Closes #3673.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• Enhancement (augments an existing feature) - feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe since breaking changes require a minor version update)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, chore(docs): fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages, and determines the next version type (patch or minor).

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• Includes component package.json meta section updates (prop, slot and event changes/updates)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (required for new features and enhancements)
• Existing test suites are passing
• CodeCov for patch has met target (all changes/updates have been tested)
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)