[BUG] [stdlib] Fix `String.format()` to use explicit byte indexing and not assume `String.__getitem__` indexes directly to byte offset.

[martinvuyk]

Bug description

Fix String.format() to use explicit byte indexing and not assume String.__getitem__ indexes directly to byte offset.

Python's String indexes by unicode codepoints (UTF-32) our String implementation currently uses UTF-8 and it's __getitem__ returns by byte offset. We need to be compliant with Python's indexing using unicode codepoints and String.format() needs to be fixed.

This is related to PR #3295 and issue #3246

@rd4com I saw it's code you commited, do you wanna take this or should I ?

Steps to reproduce

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System information

- What OS did you do install Mojo on ?
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