stats.lua: page 4 (keys): support help-like terminal printout

While --input-test is useful, and so is page 4 of stats, until now
there was no way to simply print the list in a help-like fashion.

This commit adds such printout, invoked by the script opt
stats-bindlist=yes, which uses the existing page 4 code. This prints
the list on startup and quits immediately - like any help page.

It's awkward to invoke compared to other help pages, and it does
require the stats page to be enabled (it is by default), however
it is a script-generated output, and currently there's no other
method to print a help page generated by a script.

The printout itself is performed using lua's io.write. While reliable,
it's not the standard way for mpv to print to the terminal.

Other possible printout methods are mp.msg.info - which also prints
"[stats]" prefix on each line (ugly), or forcing term-osd and setting
an osd-message which mpv will then print at the terminal - however
that's printed to stderr, and could also be subject to timing concerns
since we quit right afterwards.

In the future we can consider changing/integrating the invocation so
that mpv itself could print a help page generated by a script, thus
solving both the awkward invocation and printout-method issues.

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