Modified "jump to" code to understand address range start/end lines.
If there are multiple starts or ends at the same offset, we jump to
the first one in the set, which is suboptimal but simpler to do.
Simplified the API, embedding GoToMode in the Location object (which
is where it really needs to be, to make fwd/back work right).
Updated HTML export to grey out addresses in NON_ADDR sections.
Changed default pseudo-op strings for address regions to ".addrs" and
".adrend", after trying a bunch of things that were worse. Added
definitions for region-end pseudo-ops to Merlin32 and cc65 for display
on screen.
Added regression test 20260 for address region pre-labels.
Fixed handling of leading underscores in platform/project symbols.
These need to be escaped in 64tass output. Updated regression test
20170-external-symbols to check it.
