[clangd] Added a TextMate theme parser to the vscode extension.

Summary:
Adds a TextMate parser module to the vscode extension. It parses a theme into an array of a pair of TextMate scopes and text colors.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65738

llvm-svn: 368136