fix(postcss-merge-rules): add some missing known pseudo classes.

[ludofischer]

This is a follow-up to #1072 to re-enable merging of :visited links. I have also added non-experimental missing pseudos from #883 to the list, but merging those does not work yet (maybe the default browserslist target is just too strict?)
I have also not added pseudo classes that have to do with web components (:defined) to the list, simply because I am not sure how merging should work for custom components.

• bring back merging of :visited rules https://caniuse.com/css-sel2
• add form validation pseudo-classes https://caniuse.com/form-validation
• add :any link pseudo-class https://caniuse.com/css-any-link
• add :is pseudo-class https://caniuse.com/css-matches-pseudo

[codecov-commenter]

Codecov Report

Merging #1099 (6c5bc04) into master (c5e0a5e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1099   +/-   ##
=======================================
  Coverage   96.38%   96.38%           
=======================================
  Files         115      115           
  Lines        3594     3595    +1     
  Branches     1060     1060           
=======================================
+ Hits         3464     3465    +1     
  Misses        121      121           
  Partials        9        9           

Impacted Files	Coverage Δ
...postcss-merge-rules/src/lib/ensureCompatibility.js	98.64% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5e0a5e...6c5bc04. Read the comment docs.