Add undist script

[jamesnw]

As we migrate more keys to BCD, adding or changing keys will be more challenging. We essentially will need to "undist" the .dist file, and move the keys without comments over to the .yml file.

Some examples-

# Move compat_features keys to webgpu.yml
npm run undist features/webgpu.yml.dist

# It works for paths ending in ".yml", ".yml.dist" or without either
npm run undist features/lh

# It sorts the keys by default, but this can be disabled
npm run undist features/is -- --no-s

# You can add keys, either as multiple options or comma separated
npm run undist features/modal -- -a new.key -a another.new.key -a multiple,keys

# I'll probably run dist immediately after
npm run undist features/inert -- -a new.key; npm run dist

I'm wondering if the positional argument should be a feature id rather than a path, as this is only useful for features, and not drafts.

I opted to make this work on a single file rather than multiple to enable the add functionality, as that use case seems the most common.