[READY] Use codepoint offsets in identifier functions
`CurrentIdentifierFinished` and `LastEnteredCharIsIdentifierChar` incorrectly use byte offsets with unicode lines (`CurrentColumn` returns a byte offset and `CurrentLineContents` a unicode string). This leads to weird bugs when there is a non-ASCII character on the current line:
This is fixed by converting byte offsets to codepoint ones through the `ByteOffsetToCodepointOffset` function.
This changes the behavior of these two functions when the current column position is invalid. Both functions returned false in that case. They now return as if the current column were at the end of the line. In practice, this doesn't really matter since the position of the current column should always be valid.
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