plist parsing should (theoretically) support single quotation marks for strings

> In the process of #1295 I noticed a few other omissions related to string parsing; the major one is that the reference impl supports the use of single quotes to delimit strings. This doesn't seem to be in any of our sources, but it might be worth fixing at some point..

See https://github.com/opensource-apple/CF/blob/3cc41a76b1491f50813e28a4ec09954ffa359e6f/CFOldStylePList.c#L298

Start by locating the plist string-parsing sources in fontc and compare their behavior with the reference implementation in CFOldStylePList.c at line 298. The work is complete when plist strings delimited by single quotation marks are accepted consistently with the reference behavior.

Written by the indexing model from the issue text.