Fix exponential fontification time of backslashes in WYSIWYG-literals

[CyberShadow]

Fixes issue #64

Changes the regex introduced in #22 as follows:

• Move the first non-\ character set outside of the repeated group. As there is an identical character set at the end of the repeated group, this was redundant, and the main cause of the slowdown.
• Exclude the \ character from the character set excluding `. This was also causing fontification to take an exponential time, to a lesser degree.
• Use non-greedy matching, which should generally perform better than greedy matching.

Test file included, though it only tests for fontification time, not resulting syntax.

[CyberShadow]

Sidenote, maybe these regexen should be rewritten via rx.

[russel]

@CyberShadow Thanks for posting this pull request.