Remove the static integrity download from the `pin` and related commands.

This can't work because we modify the asset when vendoring in the application
to add a comment about the source of the asset.

Instead, we are by default delegating integrity calculation to the Rails
asset pipeline.

Fixes #308.

Improve SRI support

This make sure we are not duplicating the same regexp in multiple places
with slightly different variations.

…ter it

No need to exclude `[` or `]`.

The only option that is relevant to keep is the `preload` option.

Not all cases are kept though. If the option is set to a variable or
complex expression, it is not kept. Only simple values like `true`, `false`,
or a string/array of strings are preserved.

Keep options when updating packages in importmap

This commits introduces a new `enable_integrity!` method to the importmap
configuration, allowing users to opt-in to automatic integrity hash
calculation. This change ensures that integrity hashes
are not calculated by default, giving developers control over when to
enable this feature.

This is already opt-in for Propshaft users, but now it is also opt-in for
Sprockets users.

Make integrity calculation opt-in