Use `string.escape` capture instead of `constant.character.escape`

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Brings us one step more in line with Neovim captures. Already mistakenly used/added in languages and themes: https://github.com/search?q=repo%3Ahelix-editor%2Fhelix+string.escape&type=code. It's shorter, easier to remember, and makes more sense generally imo. I think it's easier for us to change this than to convince Neovim query authors to stop using their string.escape.