Refactor foreign script/style embedding

This commit...

1. copies most script/style embedding contexts from Svelte to add named
   contexts for each embedded script and style syntax.

2. scopes `lang="..."` attributes `meta.attribute-with-value.lang`
   to support lang value completions

   see also: sublimehq/Packages#4079

3. also accepts `language="..."` as synonym for `lang="..."`,
   as it has been seen in various other languages and has therefore also been
   added to default packages.

4. adds support for "babel" scripts

5. supports `"coffeescript"` as synonym for `"coffee"` as Svelte uses the
   former one - just to be aligned.

6. fixes scope for "Slim" templates

Author: deathaxe