[clangd] Update themeRuleMatcher when color theme changes in vscode extension.

Summary:
Add event listener that listens to configuration changes and reloads the ThemeRuleMatcher when the theme changes.

Right now it will not recolor the files, depends on the colorizer CL for that.

Reviewers: hokein, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66406

llvm-svn: 370305