You can now get logging Selection API calls

[Masayuki Nakano]

Staring from today's Nightly builds, you can get a log of Selection API
calls both internal calls and called by script.

https://bugzilla.mozilla.org/show_bug.cgi?id=1818128

For example:

```

MOZ_LOG=SelectionAPI:4,timestamp,sync ./mach run

```

The difference of log levels is:

3: Logging only called method names and parameter values
4: and the call stack of each API call of 7 callers
5: 3 and the call stack of each API call of all callers

I think that 4 is most useful for debugging, but it's not enough to get
a root callers, e.g., when you want to check whether a Selection API
call happens within synchronously dispatched event, you can specify 5 to
get deeper information.

I'd be happy if this would help your debugging especially complicated
editor apps.

--
Masayuki Nakano <masa...@d-toybox.com>
Working on DOM, Events, editor and IME handling for Gecko