Add @operator, @lifetime and @punctuation.delimiters captures for Rust

[jamesthurley]

Adds additional captures for theming rust code.

I'm uncertain about whether # belongs in the @operator capture, but I didn't see a more appropriate capture name in my brief hunt in other files. It is the prefix of an attribute_item.. suggestions welcome.

Release Notes:

• Added @operator, @lifetime and @punctuation.delimiter captures to Rust highlights file.