Reimplement compilation-mode error message detection for DMD

[CyberShadow]

This addresses the following issues:

• Column numbers were not recognized in DMD error message when -vcolumns is used. Error messages are now correctly detected with and without column numbers.
• Continuation lines, such as when indicating the instantiation point of a failed template instantiation, were not detected. They are now detected as "information" level messages.
• The previous implementation was impossible to maintain, as it was implemented as a single 235-character-long hand-optimized regular expression. The new implementation uses the rx package to specify the regular expression declaratively as lisp sexps.
• The previous implementation was not accompanied by any tests. A test file has been added.

[CyberShadow]

@russel Ping

[CyberShadow]

As there have been no objections, merging.

It's possible that this regex is not functionally a superset of the previous unmaintainable one, but perhaps someone can contribute test cases now that we have a test file.