Since we are only updating developer dependencies, we can gain enough
confidence in this change by making sure the test task exists cleanly.

We don't need to be as strict in our tests files as we are in production
code. Test code can contain English as well as some `innerHTML`
shenanigans.

Update developer dependencies

The test shows that changes to the `level` attribute of a markdown
header button aren't reflected in the output when pressing the button.

Building a header button in memory with a custom heading ("level")
causes a unexpected behaviour. Consider the following example:

```js
const headerButton = document.createElement('md-header') // The constructor is executed at this point.
headerButton.setAttribute('level', '1')                  // Has no effect since the style has already been set.
headerButton.textContent = 'h1'
document.body.appendChild(headerButton)
```

While we expect to have a header button that outputs a single hash
(`#`), we have a button that outputs three (`###`). The constructor sets
the styles based on the `level` attribute when the component is
initialised.

Changing the constructor to a `connectedCallback` function
ensures that the button styles aren't set until the button is connected
to the DOM.

`constructor` will call the `super` function automatically. We don't
have to allocate code to do that as well.

Since `constructor` can be error-prone for custom elements and keep all
the components similar, I've also changed all the constructors to
`connectedCallback` functions.

Even though we don't expect header buttons to change levels dynamically
during an application lifetime, we should ensure that it works as
expected.

So I've added an `attributeChangedCallback` to the header
button element. The callback changes the level style based on the
`level` attribute.

Use the connectedCallback function to set button styles instead of the constructor

Use chromium for karma tests

Co-authored-by: Ned Schwartz <ned@theinterned.net>

Add a .devcontainer

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