Semantic highlighting improvements for 1.43.1

[aeschli]

As mentioned in https://github.com/microsoft/vscode/wiki/Semantic-Highlighting-Overview#semantic-highlighting, the goal is to improve the first impression of semantic highlighting as well allow themes to opt-in to semantic highlighting.

• give themes a way to opt-in to semantic highlighting
• enable semantic coloring by default only the built-in themes. All other themes can be enabled by the theme author or in the user settings.
• show a notification when semantic coloring shows up the first time. The notification will point to the docs with background and information on how to configure the feature.
• fix the bugs found related to wrong token classification and color lookup.
• no longer show semantic color in imports

[MatiasOlivera]

give themes a way to opt-in to semantic highlighting

I like this feature. I think an individual config is better than a global one because it will allow us to use non-maintained themes that have semantic highlighting disabled and themes that have it enabled, without having to set the global config every time we change from a theme that uses it to one that doesn't

[MatiasOlivera]

enable semantic coloring by default only the built-in themes

It's a good default setting

[MatiasOlivera]

no longer show semantic color in imports

What are the reasons? It would be nice to know what most theme creators think, not just a few. Maybe some of us would like to use it

A configuration could be added to enable or disable that feature:

"editor.semanticHighlighting.imports": true

[aeschli]

Changes pushed.

[aeschli]

To verify:

• create TypeScript file function foo() { const x = () => {}; const y = x; }.
• If semantic color is enabled, foo, x and y all get the function color
• Open a built-in in theme, semantic color should be enabled
• verify that the very first time the semantic colors are show, a notification dialog is shown
• Install a theme from the marketplace (e.g. Atom One Dark), semantic color should be disabled.
• try

	"editor.tokenColorCustomizations": {
		"[Atom One Dark]": {
			"semanticHighlighting": true
		}
	}

and

	"editor.tokenColorCustomizations": {
		semanticHighlighting": true
	}

semantic highlighting should be enabled

• verify

var fs  = require('fs'); // require shown as function
require.resolve('fpop/bat'); // require shown as variable
Promise.resolve(null) // Promise shown as class