fix: goto_file_impl look around in chars instead of bytes

[magackame]

Fixes one more failure case missed in #12673.

Do lookaround in chars instead of bytes to prevent incorrect slicing at a random offset. Then convert to bytes for use with regex byte ranges.

[the-mikedavis]

This looks correct but I'd prefer to do byte indexing for this instead of character indexing. Ropey v2 will drop character indexing and only allow byte indexing, so this is a good opportunity to write some helper functions to solve cases like this. In this case we need a helper that shifts a byte index to its closest character boundary like str::floor_char_boundary/str::ceil_char_boundary from std. I've pushed these changes in aac0ce5...4919058