fix(postcss-merge-rules): prevent breaking rule merges

[ludofischer]

This PR fixes two related issues.

First commit, fixes: #730 by not merging the :host selector with others.
I have rebased #731 as it seems to me it fixes a legitimate problem (angular/angular-cli#18672).
Although this comment #876 (comment) says the PR is already done, the test fails on master.
There's a comment #730 (comment) mentioning the fix is not safe if the browser does not support :host but it seems the current situation is what would caus the bug.

As #999 states, merging unknown and known selectors causes the browser to ignore the whole rule, so not merging should be the safe path.
Second commit fixes #999. If the pseudo-selector is unknown and does not start with a vendor prefix, do not merge.

[codecov-commenter]

Codecov Report

Merging #1072 (5e4a3b5) into master (15235a9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1072   +/-   ##
=======================================
  Coverage   96.37%   96.38%           
=======================================
  Files         115      115           
  Lines        3588     3594    +6     
  Branches     1058     1060    +2     
=======================================
+ Hits         3458     3464    +6     
  Misses        121      121           
  Partials        9        9           

Impacted Files	Coverage Δ
packages/postcss-merge-rules/src/index.js	97.57% <ø> (-0.18%)	⬇️
...postcss-merge-rules/src/lib/ensureCompatibility.js	98.63% <100.00%> (+0.48%)	⬆️

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[ludofischer]

I think the pseudo-selector list https://github.com/cssnano/cssnano/blob/master/packages/postcss-merge-rules/src/lib/ensureCompatibility.js is incomplete, which causes deoptimizations after this pull request. For example, the list is missing visited, so we get:

blockquote cite a, blockquote cite a:visited {color: #555} 

becomes

blockquote cite a { color: #255 }blockquote cite a:visited {color: #555}

That's because before it sees :visited as an unknown pseudo-selector, so it does not merge it anymore.