Add token/symbol for inactive code

Currently, extensions have an odd way of dimming or coloring inactive code. They add non-standard a CSS class and the class dims the code. The opacity and colors are configured in the extension settings instead of in the standard place for colors in VS Code: tokenColorCustomizations: textMateRules.

// Here's what this may look like in VS Code if the user wants to configure it.
"editor.tokenColorCustomizations": {
	"textMateRules": [
		{
			"scope": "inactive",
			"settings": {
				"opacity": 0.5
			}
		}
	]
},

Here are some example of how this works already.

• vscode.typescript-language-features
• ms-vscode.cpptools

Start by comparing the inactive-code handling in the linked vscode.typescript-language-features and ms-vscode.cpptools examples. Then identify the language-server-protocol location where a token or symbol for inactive code would belong; done means extensions can represent inactive code through the standard tokenColorCustomizations textMateRules configuration.