Add copyright headers in a few files where they were missing.
Trim trailing blank comment line where present, for consistency with
other engine code.
Use the standard libtxt copyright header in one file where it differed
(extra (C) and comma compared to other files in libtxt).
This also amends tools/const_finder/test/const_finder_test.dart to look
for a const an additional four lines down to account for the copyright
header added to the test fixture.
This removes TextRange from the framework and moves it to the engine, in preparation for using it to return text ranges from the text extent APIs, like Paragraph.getWordBoundary instead of a List<int>.
Also added new tests for TextRange.
