Fix for escape character matching

[juanitogan]

The current language file uses this for matching escape sequences:

<string>\\.</string>

If you turn off the PICO-8 theme, you will see that decimal and hex escape sequences aren't fully colorized. This fixes it (adapted from the latest Lua highlighter):

<string>\\([abfnrtv\\"']|\r?\n|\n\r?|\d\d?\d?|[xX][0-9a-fA-F]{2})</string>

Replace both occurrences in PICO-8.tmLanguage.