feat(contented): ability to customize unified plugins

[fuxingloh]

What this PR does / why we need it:

Make it easier to customize content processor with subset of plugins.

[netlify]

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[codecov]

Codecov Report

Merging #629 (afe6353) into main (0551e1b) will increase coverage by 6.72%.
Report is 70 commits behind head on main.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
+ Coverage   85.00%   91.72%   +6.72%     
==========================================
  Files          10       13       +3     
  Lines         420      544     +124     
  Branches       73      100      +27     
==========================================
+ Hits          357      499     +142     
+ Misses         63       45      -18     

Files	Coverage Δ
...ented-pipeline-jest-md/src/JestMarkdownPipeline.ts	100.00% <100.00%> (ø)
...ages/contented-pipeline-md/src/MarkdownPipeline.ts	86.84% <100.00%> (+1.54%)	⬆️
...ages/contented-pipeline-md/src/UnifiedProcessor.ts	92.30% <100.00%> (+4.80%)	⬆️
...contented-pipeline-md/src/plugins/RehypeHeading.ts	93.54% <ø> (+6.45%)	⬆️
...contented-pipeline-md/src/plugins/RehypeMermaid.ts	93.75% <ø> (+50.00%)	⬆️
...ntented-pipeline-md/src/plugins/RemarkCodeblock.ts	100.00% <100.00%> (ø)
...d-pipeline-md/src/plugins/RemarkDirectiveRehype.ts	100.00% <100.00%> (ø)
...es/contented-pipeline-md/src/plugins/RemarkLink.ts	100.00% <100.00%> (+28.57%)	⬆️
...ages/contented-processor/src/ContentedProcessor.ts	88.46% <100.00%> (+2.04%)	⬆️
...s/contented-pipeline-md/src/plugins/RehypeShiki.ts	96.77% <96.77%> (ø)
... and 2 more