When a Ruby file contains multibyte characters, the go to definition and
hover features do not work correctly. This is because the index building
and document referencing logic does not properly handle multibyte
characters when calculating offsets.
This commit fixes the issue by:
•Updating index building to use character offsets instead of byte
offsets
•Modifying document referencing to properly handle multibyte characters
when mapping between positions and offsets
•Adding test cases to verify go to definition work with multibyte
characters

When a Ruby file contains multibyte characters (like Japanese, Chinese,
emoji, etc), the go to definition and hover features do not work
correctly. Because the document referencing logic does not properly handle
multibyte characters when calculating offsets.

This commit fixes the issue by:
*Modifying document referencing to properly handle multibyte characters
when mapping between positions and offsets
*Adding test cases to verify go to definition work with multibyte
characters