Further Emacs highlighting improvements

Two simple improvements to Julia's syntax highlighting.

The first fixes #8259, ensuring code like `@swap!(x, y)` is highlighted correctly. Previously, the exclamation mark was not highlighted as part of the macro invocation.

The second commit improves function highlighting. If a function is defined using the terse assignment syntax, e.g. `foo(x, y) = 1`, this is now highlighted as a function definition. It's not perfect, but it handles the vast majority of functions I've tested it on.