🔧 More improvements for directive option parsing

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.13%. Comparing base (8614eca) to head (8cd5441).

Files	Patch %	Lines
myst_parser/parsers/options.py	92.30%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #919 +/- ## ========================================== - Coverage 90.14% 90.13% -0.02% ========================================== Files 24 24 Lines 3400 3415 +15 ========================================== + Hits 3065 3078 +13 - Misses 335 337 +2 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/919/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.13% <94.28%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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