Parser work

[xyz65535]

This PR is related to #52

This PR does the following:

• Identifying specific problems with the parser
• Making grammar improvements
• Verifying that these improvements increase roundtrip capability for the rice-2023 document
• Ensuring all existing tests still pass to avoid introducing bugs

Regarding rice-2023 document roundtripping: without this pull request, only 18 out of 48 files were parsed, meaning just 37% were processed, and 655 lines remained unparsed. Now, all 48 files are parsed without breaking, leaving only 104 lines unparsed. Adding all together, it used to be that 1,746 lines were unparsed before, so this pull request, as of today, makes a 94% reduction in unparsed lines.

Metanorma PR checklist

• [ ] Breaking changes (list related PRs)
• [ ] Documentation update required (create task for this)
• [ ] External dependency introduced (documentation update need)
• [ ] Gem with native library introduced

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.35%. Comparing base (a32cdeb) to head (e4248c5). Report is 17 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #117 +/- ## ========================================== + Coverage 98.34% 98.35% +0.01% ========================================== Files 46 46 Lines 1327 1337 +10 ========================================== + Hits 1305 1315 +10 Misses 22 22 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.