chore: separate source from state

[azjezz]

Not much of a performance gain, but it makes lexer state easier to work with.

benchmarks:

before:

 Time (mean ± σ): 2.519 s ± 0.017 s [User: 2.611 s, System: 0.225 s] Range (min … max): 2.501 s … 2.551 s 10 runs 

after:

 Time (mean ± σ): 2.451 s ± 0.007 s [User: 2.561 s, System: 0.190 s] Range (min … max): 2.443 s … 2.462 s 10 runs