#163967 css alias

[SStranks]

In conjunction with Pull Request #197033 on the vscode repository.

I reworked the previous merged code for this feature so that it now handles single-root, multi-root, and mono-repo project structures.

To clarify, aliases for css/scss are specified in the .vscode/settings.json under 'importAliases' for each language as before - I have updated the tool-tip descriptions in the package.nls.json file, and made code changes to cssServer to accommodate the functionality in the current PR - see PR 197033 above.

Functionality

• Settings files/aliases at the workspace root take priority over any nested settings files. This allows avoiding directory tree lookup for settings files if a valid alias is discovered there first.
• If no settings/alias is found at the workspace root; attempt lookup for .vscode/settings.json from document towards workspace root.
• If lookup finds settings file; cache the path and cache the aliases for that document to avoid repeat lookups.
• Added Public clearCache() method to CSSNavigation; to be called when settings.json changes (see limitations section below)

Changes

• Added functionality above.
• Updated: navigation tests.
• Added: test fixture files

Limitations

• clearCache() method only fires when settings files tracked by vscode are changed. Settings files discovered by lookup in cssNavigation are not covered. I didn't see a straightforward way to implement this; vscode cssServer imports connection.onDidChangeConfiguration from vscode-languageserver, and handles the watcher. Willing to make further changes if someone can suggest how/where to look.