feat(#8): char is LOWERCASE_LETTER

[h1alexbel]

closes #8

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PR-Codex overview

This PR focuses on modifications to the parsing rules in src/program.pest and src/parser/fsl_parser.rs, specifically adjusting the definitions for login and char, as well as updating test cases to reflect new valid and invalid inputs.

Detailed summary

• In src/program.pest:
  • Changed -login to +login definition.
  • Updated -char to +char to only allow LOWERCASE_LETTER.
• In src/parser/fsl_parser.rs:
  • Updated test cases to change valid input from "foo123" to "foo" and added invalid inputs "X" and "XYZ".
• Removed comments regarding prohibiting upper-case letters in references.

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[h1alexbel]

@rultor merge

[rultor]

@rultor merge

@h1alexbel OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@h1alexbel Done! FYI, the full log is here (took me 42min)