This adds the ability to mark sources emitted into a source map as
ignore-listed, which provides a hint that debuggers can utilize when
dealing with these sources[^1].

The `ignoreList` option can be passed to a `MagicString` now, and will
be considered when generating a source map for it. In addition that bit
will also be considered when the `MagicString` is added as source to a
`Bundle` (with the possibility to override the value via a parameter to
`addSource`). The `x_google_ignoreList` field in the source map will
only be emitted if at least one of its sources was explicitly marked
with `ignoreList: true`. Otherwise - and primarily for backwards
compatibility with the existing ecosystem - no `x_google_ignoreList`
field is emitted.

Fixes Rich-Harris#241

[^1]: https://developer.chrome.com/blog/devtools-better-angular-debugging