Allow non-parsable code in headers

[lorenzwalthert]

Closes #831.

[codecov-commenter]

Codecov Report

Merging #832 (13e3523) into master (027d5b1) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 13e3523 differs from pull request most recent head 74b1f1f. Consider uploading reports for the commit 74b1f1f to get more accurate results

@@           Coverage Diff           @@
##           master     #832   +/-   ##
=======================================
  Coverage   90.53%   90.53%           
=======================================
  Files          47       47           
  Lines        2451     2451           
=======================================
  Hits         2219     2219           
  Misses        232      232           

Impacted Files	Coverage Δ
R/transform-code.R	94.44% <100.00%> (ø)

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[github-actions]

This is how benchmark results would change (along with a 95% confience interval in relative change) if 13e3523 is merged into master:

• cache_applying: 0.09s -> 0.03s [-69.75%, -62.97%]
• cache_recording: 1.01s -> 0.99s [-2.36%, -1.48%]
• without_cache: 2.56s -> 2.54s [-1.02%, -0.54%]

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