This PR adds support for field selectors (`.<field>`) in the lint plugin
API. This is supported in ESLint as well, but was missing in our
implementation.
```css
/* Only search the test expression of an IfStatement */
IfStatement.test
```
Fixes https://github.com/denoland/deno/issues/28314
The selector splitting code that's used for JS linting plugins didn't
properly account for selectors being a single character. This can happen
in the case of `*`.
Instead of comparing against the length, we'll now check if the
remaining string portion is not empty, which is more robust. It also
allows us to detect trailing whitespace, which we didn't before.
