Enabling high contrast dark theme on all platforms + hc-dark improvements

[bgashler1]

Addresses #2711

• Enables the hc-dark theme on all platforms.
• Improves color contrast to AAA standard on many elements that weren't previously meeting it.
• Resolves visual bugs, such as synthetic-focus misplacement and action bar duplicate icons
• Differentiates the selection of text from current line and find/word matching
• Works well in Mac OSX even when OS-level accessibility features are enabled, such as increasing contrast.

NOTE:

A CSS rule used (mix-blend-mode: difference;) currently works in all browsers but Opera, IE and Edge, so it will not be ideal for the online Monaco editor without suitable shims. Let me know if we need to address this; we can do that now or in the future.

[alexdima]

LGTM

[alexdima]

@bgashler1 It is possible to write conditional browser selectors for ie or ff:

e.g.:

.monaco-editor.ff .inputarea,
.monaco-editor.ie .inputarea {
    width: 0;
    height: 0;
}