Add sync transform if w/ only explicit plugins

[talatkuyuk]

Initial checklist

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

Description of changes

I added a synchronous transformer if "options.plugins" doesn't contain a string.

I've been using react-markdown which supports only synchronous transformers. The plugins that I used in the plugin list in my project is about 20, the only async transformer was remark-textr which caused a problem. I created a custom plugin in my project, but wanted to share with the community.

I added a test and also tested in with ReactMarkdown in my project.

I don't believe it's necessary to add this to the README since it doesn't affect users. This does not require any breaking change.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (429e2ca) to head (5cd39d6).

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Additional details and impacted files

@@            Coverage Diff            @@
##              main       #40   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           82       105   +23     
=========================================
+ Hits            82       105   +23     

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[wooorm]

Sounds like a good idea, thank you!

[talatkuyuk]

All your suggestions are applied. Hope it is fine.

[wooorm]

Released! Thank you!