Escape colons in definition term text

Fixes this test case:
echo '<dl><dt>foo: bar</dt></dl>' | node parse --html2wt
;<nowiki>foo: bar</nowiki>

No change in parserTests, so need to add a test for this.

Bug: 47747
Change-Id: Ibab4c46f2c517bcd9398713e6dee3f7c0188144e
Merge "Escape colons in definition term text"