CSS Processor string token values mishandle backslash-EOF

[sirreal]

In CSS <string> token values, WordPress\DataLiberation\CSS\CSSProcessor::get_token_value() reports an incorrect token value when \ is at the end of input.

See the rules for consuming a string token:

Repeatedly consume the next input code point from the stream:

…

U+005C REVERSE SOLIDUS (\)

• If the next input code point is EOF, do nothing.
  …

Related to #222.

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Demos

$p = WordPress\DataLiberation\CSS\CSSProcessor::create( "'string\\" );
$p->next_token();
assert( $p->get_token_type() === WordPress\DataLiberation\CSS\CSSProcessor::TOKEN_STRING );
assert( $p->get_token_value() === 'string', 'Expected "string", got ' . var_export( $p->get_token_value(), true ) );

Will throw AssertionError: Expected "string", got 'string�'.

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Example of string value equality

"AB" === "AB\{{END-OF-INPUT}}

This should render with a fancy font, <i>not monospace</i>.
<style>
/* Note that the two "AB" strings match. */
body { font-family: 'AB', monospace;}
@font-face {
  src: url(https://fonts.gstatic.com/s/lobster/v32/neILzCirqoswsqX9zoKmM4MwWJU.woff2) format('woff2');
  font-family: 'AB\