refactor: execute option

[alexander-akait]

BREKING CHANGE: the exec option was renamed to the execute option

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Rename exec to execute

Breaking Changes

Yes

Additional Info

You can't rely on postcss-js name of parser, because parser can be Function and check will not work, you need to set the option to true value if you want to use postcss-js

[codecov]

Codecov Report

Merging #464 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   97.29%   97.26%   -0.04%     
==========================================
  Files           5        5              
  Lines         222      219       -3     
  Branches       62       61       -1     
==========================================
- Hits          216      213       -3     
  Misses          6        6              

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)
src/utils.js	95.91% <100.00%> (-0.09%)	⬇️

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