Fix to set `vfile.value` if a vfile@5 is given

[wooorm]

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

vfile@5 changed from the contents to the value field. Given that not all of the ecosystem has updated yet, and that some version mismatches will occur for the foreseeable future, setting bothcontents*and*value` in those cases will hopefully alleviate some of the struggles with updating.

Related to remarkjs/remark-frontmatter#16.

[codecov-commenter]

Codecov Report

Merging #143 (2244176) into v9 (60de570) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                v9      #143   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          456       460    +4     
=========================================
+ Hits           456       460    +4     

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

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