support `\u` and `\u{...}` syntax

[BurntSushi]

c.f. BurntSushi/ripgrep#702 and microsoft/vscode#39404

I was historically not too motivated to add this since Rust string literals provide this syntax, but regexes can be written outside the context of Rust code, and in that case, it seems like good sense to just support the syntax that folks might expect.

I expect this to be a new feature in the upcoming regex-syntax rewrite.