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