feat: first-line-regex to detect languages

[sigmaSd]

Add a new method to detect a language: first-line-regex

This is taken from tree sitter https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-detection (there is also content-regex)

Vim also have something similar

This is useful for files that usually don't have an extension but have a known layout: strace output, tree sitter tests, etc..

[pascalkuthe]

clsoing this one out as stale. This kind of thing is fairly niche and has an easy workaround (:language). I think this can be added as a plugin in the future. Thank you for contributing!