feat(config): add enum options for styles

[hsqStephenZhang]

Background

When typing or selecting styles in vscode config editor, the usable options are not shown due to missing of enum field in each style field in properties in package.json, which could cause confusions for new comers.

Fix

Add enum of all preset style options shown in https://clang.llvm.org/docs/ClangFormat.html#standalone-tool

TODOs

• if we should enable this enum for each style field? (which could be lengthy)

[hsqStephenZhang]

After the change: