[Bindings/Odin] Improve example text measurement function

[rats159]

This PR changes the example Odin text measurement function to

1. have proper UTF-8 support using odin's rune type and iterating the string by codepoint, rather than by byte
2. support clay's letter spacing configuration
   The implementation was heavily inspired by raylib's own MeasureTextEx function

[nicbarker]

Hello, thanks for opening this! 🙂
For context, the reason I implemented the original measuretext function by hand is that the GetGlyphIndex function does a linear scan through a potentially large array which can be very slow, especially if you're only dealing with english. Could we split this UTF-8 supporting measuretext into a seperate function, and still default to the ASCII one?