Improve syntax highlighting of keywords by creating groups for
declarations and import/export.

### Release Notes:
- Highlight declarations with `keyword.declaration`:
  - const
  - let
  - var
  - function
  - class
  - enum
  - interface
  - type

- Highlight imports and exports with `keyword.import`:
  - export
  - from
  - import

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Co-authored-by: Gaauwe Rombouts <mail@grombouts.nl>