add linter for css variable names

[aeschli]

Add a linter that checks that all used variable names that appear in css files are known.

The linter is part of hygiene or can be run with yarn stylelint

All known variables are in a file vscode-known-variables.json. To validate that vscode-known-variables.json is up-to-date, run ./scripts/test-documentation (that test also verifies that all color variables are documented)

This linter just uses regexp on line content, it doesn't understand CSS (in particular comment tokens). In a a follow up PR I have a solution that's based on stylelint.

The PR also fixes some wrongly named variables.

[aeschli]

@hediet in the merge editor css, I removed references to colors that do not exist.

.monaco-workbench .merge-editor .merge-editor-block:not(.handled) {
	background-color: var(--vscode-mergeEditor-conflict-unhandledUnfocused-background);
}
.monaco-workbench .merge-editor .merge-editor-block.handled:not(.focused).conflicting {
	background-color: var(--vscode-mergeEditor-conflict-handledUnfocused-background);
}
.monaco-workbench .merge-editor .merge-editor-block.handled.focused.conflicting {
	background-color: var(--vscode-mergeEditor-conflict-handledFocused-background);
}

Please decide if you want to bring them back and add color definitions

@connor4312 Same for the testing.css: --vscode-testingColorRunAction is not a color (yet)