Fitb literal strings

[dbrianwalton]

Targeted to address #2460

Description of changes:

• Added a template in pretext-text-utilities.xsl that takes a string and escapes regular expression token characters so that they would match literally in a regular expression comparison. \ . ^ $ * + ? { } [ ] ( ) |
• When a FITB answer checker for a string involves @use-answer="yes", it compares with the regex-escaped answer string instead of the answer string itself
• The original code used xsl:choose for a case that no answer checker was provided for the correct response so that it used the provided answer. This introduced a second area that would have needed the correction, so instead we used the mode="exercise" during assembly-stage to insert a missing <test correct="yes"> element so that the same branch of logic is followed whether the author provided the element or not.

Although I originally expected I needed exslt for regular expressions, the standard str:replace method using search/replace pairs did the job fine.

[dbrianwalton]

sec-fitb-string-tests.xml

This file contains example exercises that would not have worked previously that now work. (Claude helped construct the examples.)