Simplify SavePosition/RestorePosition functions

[dmitriplotnikov]

Simplifies position saving and restoring in the lexer by replacing the lexerPosition struct with a simple int32 offset and removing unused tracking fields.

Key changes:

• Updated SavePosition() to return int32 (l.pos).
• Updated RestorePosition(pos int32) to restore l.pos and reset l.err.
• Removed lexerPosition struct definition.
• Removed unused atEnd and done fields from lexer.