Add support for Semantic Highlighting (Tokens)

[icebr9]

Is your feature request related to a problem? Please describe.
Implement semantic tokens so theme makers can apply Svelte-specific styling in their themes. (This is a feature since VSCode 1.43)
https://code.visualstudio.com/api/language-extensions/semantic-highlight-guide#theming
Following addition, guidelines/docs for theme makers should be added to the plugin description so they can target these tokens.

Describe the solution you'd like
Sample implementation can be found here, https://github.com./Microsoft/vscode-extension-samples/tree/master/semantic-tokens-sample

Describe alternatives you've considered
Make it a seperate optional plugin instead of integrating into Svelte Beta.