OOML Writer: Add low line char ("_") to isBarChar

[Hagb]

"_" is used for \underline in symbolMapOverrides. "_" should be regarded
as bar char, or \underline{xxx} will be written as \underset{\_}{xxx}
in OOML.

Fix jgm/pandoc#8152

[jgm]

Thanks for this. CI tests are failing. Take a look at the failing test cases and make sure the actual output looks correct. If so, you can update the expected test results to the actual results using make TESTARGS=--accept (or pass in --accept to the test suite if you're running cabal test or stack test manually). More on this in README.md.

[Hagb]

Sorry for the late reply. The output looks correct and the test case has been updated.

[jgm]

Great, thanks.