Add syntax definition for ACPI Source Language

This PR adds a highlighting definition for ACPI Source Language (state of ACPI 6.5 A) and a wrapper for other ACPI Source files that all have the same file extension.

I use this for highlighting a 3 MiB DSDT with 69600 lines (!), haven't noticed any problems. ksyntaxhighlighter6 takes around 180ms for that file.

Draft because:

• Is this kind of mechanism for handling the rather unspecific *.dsl extension ok?
• I extraced the keyword list from acpica/iasl source code and thus made this BSD-3-Clause licensed. Is that ok?
• Is there a way to use the region="Comment" from acpi-asl.xml in acpi-dsl.xml? Currently this doesn't work.
• Are unit tests for this necessary?

Example: