Updated plug-able pipeline functions to be async

[rkesters]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

unified-engine supports plug-ins, for various steps in the pipeline, for example stringify. However, currently the plug-ins must be sync; this is not a necessary constraint as the pipeline is managed by trough which natively handles middle-ware that returns either a promise or a value. Hence, allowing plug-able pipeline functions to return promises will make i easier to implement plug-ins.

Addresses #62

[codecov-commenter]

Codecov Report

Merging #63 (d64caf5) into main (19a498e) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #63   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         2515      2515           
=========================================
  Hits          2515      2515           

Impacted Files	Coverage Δ
lib/file-pipeline/parse.js	100.00% <100.00%> (ø)
lib/file-pipeline/stringify.js	100.00% <100.00%> (ø)
lib/file-pipeline/transform.js	100.00% <100.00%> (ø)

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[rkesters]

@wooorm not sure why this failed on windows, can you take a look and advise.

[ChristianMurphy]

the workflow has been getting stuck at https://github.com/unifiedjs/unified-engine/blob/19a498e9c42bf103b885b969e874c2790d4ead81/.github/workflows/main.yml#L24

lately, changing that line from node to lts/gallium usually resolves the error

[wooorm]

This behavior is intentional: https://github.com/unifiedjs/unified-engine/issues/62#issuecomment-1060396465

[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.