ytdl_hook: make path and json available to other scripts

It's useful for user scripts to be able to use the same ytdl binary that
ytdl_hook uses without having to replicate ytdl_hook's process of
searching for the ytdl binary.

Some user scripts might also find it useful to be able to access ytdl's
json output that the ytdl_hook already receives, sparing user scripts
from having to make a duplicate ytdl binary invocation to get the json
output.

Providing just the json output is not enough though, as ytdl doesn't communicate
errors though it -- if an error occurs, ytdl provides no json output and instead
prints to stderr. So without stderr, there is no way for user scripts to figure
out why ytdl has failed: no such username / video id, the channel is not live
yet, etc. Because of that, the entire result of the subprocess call is provided
to the user scripts, containing stdout (json), stderr, ytdl's exit code, etc.

player/lua/ytdl_hook.lua

@@ -993,12 +993,16 @@ local function run_ytdl_hook(url)
         end
 
         ytdl.searched = true
+
+        mp.set_property("user-data/mpv/ytdl/path", ytdl.path or "")
     end
 
     if result.killed_by_us then
         return
     end
 
+    mp.set_property_native("user-data/mpv/ytdl/json-subprocess-result", result)
+
     local json = result.stdout
     local parse_err = nil
 
@@ -1202,3 +1206,7 @@ mp.add_hook("on_preloaded", 10, function ()
         chapter_list = {}
     end
 end)
+
+mp.add_hook("on_after_end_file", 50, function ()
+    mp.del_property("user-data/mpv/ytdl/json-subprocess-result")
+end)