Allow setting org_todo_keyword_faces directly with `@org.keyword.face.*` highlight groups

[folliehiyuki]

Each @org.keyword.face.<name> highlight group is only considered in orgmode's highlight specs when <name> is defined inside org_todo_keyword_faces option with at least 1 attribute. Since the option is static, it cannot adapt to users changing Neovim colorschemes at runtime.

My workaround for now is to put a temporary face value (.e.g :weight bold) for each keyword and redefine the corresponding @org.keyword.face.<name> highlight groups in my colorscheme file.

It'd be nice if orgmode can register a single @org.keyword.face.* highlight group unconditionally for each entry in org_todo_keywords option, first considering the highlight settings of org_todo_keyword_faces, and if not available, linking to either @org.keyword.todo and @org.keyword.done by default. The users are free to override the highlight groups in their colorscheme configuration.