CSS Minification: Decline to minify files containing unicode escapes like \f178

[mreishus]

We found a bug that happens when minifying files like WP's dashicons.css.

It uses unicode escapes like so:

.dashicons-sticky:before {
	content: "\f537";
}

.dashicons-store:before {
	content: "\f513";
}

.dashicons-superhero-alt:before {
	content: "\f197";
}

The current engine turns these into unicode codepoints, then renders them in utf-8 bytes. If the browser decides to interpret the file as ASCII, or another non-utf8 encoding, then the user will see 文字化け (mojibake) instead of their icon.

It has been difficult to find an engine that hits all three points of:

• Supports modern CSS syntax and is actively maintained
• Does not reencode unicode escape sequences
• Is performant

While we evaluate other options, this PR changes the behavior to skip minifying files that contain unicode escape sequences. While this gives up some minification benefits, it ensures the user will always see icons as intended.

[mreishus]

We probably don't need to do this anymore with #51 also covering this problem

[mreishus]

We decided to go with CSSNano + Lite Preset.