Fix style sheet parsing

This is for #9875 .

When minimizing style sheets for parsing, limit string removal to content property.

Added unit tests to CSSUtils and Quick View.

So if someone had `content:  ".uploading-item{style: 'background: linear-gradient(to right, #e8eef3, {{this.percent}}%, transparent 100%;'}" in a file, the new form of this code would get into an infinite loop. That's obviously a lot less likely than the problem described in this bug.

As a fix then, we could call `CSSUtils.reduceStyleSheetForRegexParsing` only on CSS, SCSS, SASS, Stylus and LESS files or only do the QuickView for certain kinds of files. HAML is like HTML, so QuickView there does seem reasonable... so maybe we limit the stripping of strings to only stylesheet-like files?