1) Define all commands in one file.
2) Add logic to make 'jco' print current pc by default.
3) Add a comment to explain how to load the lldb_commands.py file.
4) Minor refactorings.

R=ahaas@chromium.org
No-Try: true

Bug: v8:7754
Change-Id: I553f2ce4cefedad05466c692a8665a570372b76a
Reviewed-on: https://chromium-review.googlesource.com/1127892Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54329}