Use the major mode's syntax table directly.

So far, we assumed that `[]` `()` `{}` are always the delimiters, and the only delimiters, we want to highlight. But this is not true. For example, in Pascal, `{}` are comment delimiters, and in C++, `<>` serve as brackets in template syntax. The respective major modes get this right, we don't, so rely on them. Code simplification is an added benefit.

Note: currently it's probably buggy and certainly incomplete. For example, it doesn't respect `rainbow-delimiters-highlight-TYPE-p` yet.