support qmd file format, and treat it internally as R markdown

[lorenzwalthert]

Closes #913.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (95648d7) to head (5df9c8a).
⚠️ Report is 829 commits behind head on main.

Files with missing lines	Patch %	Lines
R/addins.R	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #931      +/-   ##
==========================================
+ Coverage   90.48%   90.50%   +0.01%     
==========================================
  Files          47       47              
  Lines        2585     2600      +15     
==========================================
+ Hits         2339     2353      +14     
- Misses        246      247       +1     

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 20f4a45 is merged into main:

• ❗🐌cache_applying: 29.4ms -> 30.6ms [+1.25%, +7.03%]
•   :ballot_box_with_check:cache_recording: 1.31s -> 1.31s [-1.03%, +0.98%]
•   :ballot_box_with_check:without_cache: 3.49s -> 3.49s [-0.85%, +0.86%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if ce3f04f is merged into main:

•   :ballot_box_with_check:cache_applying: 25.9ms -> 25.8ms [-1.4%, +0.46%]
•   :ballot_box_with_check:cache_recording: 1.12s -> 1.11s [-0.92%, +0.1%]
•   :ballot_box_with_check:without_cache: 3.03s -> 3.04s [-0.22%, +0.66%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if a49c32a is merged into main:

•   :ballot_box_with_check:cache_applying: 24.5ms -> 24.4ms [-2.25%, +1.35%]
•   :ballot_box_with_check:cache_recording: 1.14s -> 1.08s [-15.97%, +5.21%]
•   :ballot_box_with_check:without_cache: 2.87s -> 2.86s [-1.29%, +0.62%]

Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 38923f3 is merged into main:

•   :ballot_box_with_check:cache_applying: 35.4ms -> 35.6ms [-3.21%, +4.02%]
•   :ballot_box_with_check:cache_recording: 1.69s -> 1.67s [-1.73%, +0.26%]
•   :ballot_box_with_check:without_cache: 4.61s -> 4.62s [-1.3%, +1.5%]

Further explanation regarding interpretation and methodology can be found in the documentation.