Adjustments to the markup of a few tokens

- Markup `=` in the same way in `x = 1`, `function (x=1)` and `f(x=1)`;

- Recognize operators `::` and `:::`;

- Treat `NULL` like `NA` and `TRUE` rather than keywords.

Besides,

- `SLOT` is currently formatted like formal argument names. Is this intended? How about  leaving it out like `SYMBOL`, e.g. in `foo$bar`?

- Would it look better if we supplement `.operators` with (some of) `[`, `]`, `LBB`, `(`, `)`?