Compact style mutation fixes and improvements (#1268)

* Don't use the CSSOM when there's `var()` present as it fails badly https://github.com/rrweb-io/rrweb/pull/1246

* As the CSS Object Model expands out shorthand properties, do a check on the string length before choosing which format to go for

 - this approach allows 'var()' in a styleOMValue as it's only a problem when combined with a shorthand property
 - before this change background:black; was getting expaned to 10 OM properties as follows:

'style': {
    'background-color': 'black',
    'background-image': false,
    'background-position-x': false,
    'background-position-y': false,
    'background-size': false,
    'background-repeat-x': false,
    'background-repeat-y': false,
    'background-attachment': false,
    'background-origin': false,
    'background-clip': false
}

* Updates to remainder of tests based on refined compact style mutations

* Apply suggestions from code review by: Justin Halsall <Juice10@users.noreply.github.com>

---------

Authored-by: eoghanmurray <eoghan@getthere.ie>

.changeset/clean-plants-play.md

@@ -0,0 +1,9 @@
+---
+'rrweb': patch
+'@rrweb/types': patch
+---
+
+Compact style mutation fixes and improvements
+
+- fixes when style updates contain a 'var()' on a shorthand property #1246
+- further ensures that style mutations are compact by reverting to string method if it is shorter