[lldb][bazel] Add HighlighterDefault, rename ClangHighlighter targets (#182693)
Rename `PluginClangHighlighter` to `PluginHighlighterClang` for
consistency with the directory-based naming convention, add the new
`PluginHighlighterDefault` library, and register both `HighlighterClang`
and `HighlighterDefault` in `DEFAULT_PLUGINS`.