Debugger: improve command help text

Improve the help text of the db, dw, ds, dl, and string commands.

This is accomplished by
* Splitting CliDumpMemoryCommand into CliDumpMemoryCommand and
CliDumpStringCommand
* Moving code shared between the two into CliContext
* Removing functions in CliContext that were replaced by this movement
* Allowing customizing the help text for CliDumpMemoryCommand for each
command it represents
* Changing the help text for CliDumpStringCommand

Change-Id: If4f9e0c20f00f3e3d6c6769216fabb3160aea0a4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6332
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>