ytdl_hook.lua: always specify --write-srt

Currently by default mpv sets --all-subs when calling yt-dlp for JSON
info. --all-subs is an option that simply expands to --sub-langs all
--write-subs. --write-subs is special that without it yt-dlp will no
longer extract any subtitle info.

mpv's yt-dlp hook has a logic that if user specify any sub-langs in
--ytdl-raw-options, it no longer uses --all-subs. Unfortunately, this
means unaware users will not get any subtitle any more until they study
the mpv internal code and add their own --write-subs.

This PR always specifies --write-srt, an alias of --write-subs, in the
hook, so users don't have to. The alias is used to maintain
compatibility with youtube-dl.

Fixes issue #14818

player/lua/ytdl_hook.lua

@@ -966,8 +966,11 @@ local function run_ytdl_hook(url)
     end
 
     if allsubs == true then
-        table.insert(command, "--all-subs")
+        table.insert(command, "--sub-langs")
+        table.insert(command, "all")
     end
+    table.insert(command, "--write-srt")
+
     if not use_playlist then
         table.insert(command, "--no-playlist")
     end