Add a size limit when reading files

Added --max-size flag which defaults to 2MB. After this is reached the command will exit. Should help with staying under context limits as well as stopping when hitting large files.

$ code-to-gpt -c --include "**/*.go" --max-size 10000
2024/11/28 00:47:45 parsing files: 
...maximum size limit reached at code-to-gpt/parser/run.go (2345 bytes).
...current limit (10000 bytes), maybe set --max-size to a value higher than that
exit status 1