Allow semantic token styles with undefined foreground and styles

[aeschli]

When we style semantic tokens, some style properties (foreground, bold, italic, underline) can be undefined.

• when ColorTheme.getTokenStyleMetadata uses ColorId.None as foreground, we want the foreground of the underlying TextMate tokens to be used.
• we need to extend the API to express that bold, italic, underline are set, not set or undefined.

[alexdima]

Here is a good sample:

• sample.ts:

const zz = 123;

• settings.json

    "workbench.colorTheme": "Monokai Dimmed"
	"editor.tokenColorCustomizationsExperimental": {
		"variable": { "fontStyle": "underline" }
	},
	"editor.tokenColorCustomizations": {
		"textMateRules": [
			{
				"scope": "variable.other.constant.ts",
				"settings": { "foreground": "#63ee47" }
			},
		]
	}

• when having "editor.semanticHighlighting.enabled": false:
  

• when having "editor.semanticHighlighting.enabled": true:
  

Ideally, the color would be taken from the TM token and the underline should be taken from the semantic token: