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

[lorenzwalthert]

Closes #913.

[codecov-commenter]

Codecov Report

Merging #931 (68e846d) into main (aa32091) will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #931      +/-   ##
==========================================
+ Coverage   90.48%   90.51%   +0.02%     
==========================================
  Files          47       47              
  Lines        2585     2593       +8     
==========================================
+ Hits         2339     2347       +8     
  Misses        246      246              

Impacted Files	Coverage Δ
R/addins.R	14.94% <0.00%> (ø)
R/set-assert-args.R	92.85% <100.00%> (+0.17%)	⬆️
R/transform-code.R	94.44% <100.00%> (ø)
R/ui-styling.R	100.00% <100.00%> (ø)
R/utils-files.R	100.00% <100.00%> (ø)

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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.