Improve CharSequences extension
Improve CharSequences extension

lib: Added the `first`, `last` and `indices` methods to the `CharSequences` extension.
lib: Added the `contains`, `count` and `split` methods to the `StringCodePoints` extension.
lib: Moved the `|_|` operator methods to the `CharSequences` extension.

Clashsoft/Dyvil,Clashsoft/Dyvil