Lua API: allow smooth 'hud_elem_type' transition phase

[SmallJoker]

Migrating this field name is annoying (need protocol version checks), hence silently accepting exact duplicates allow a convenient transition phase.

Opened because PRs like minetest-mods/areas#78 are bound to happen sooner or later, thus making the transition phase easier seems like a good idea.

To do

This PR is Ready for Review.

How to test

1. deprecated_lua_api_handling = log
2. use https://github.com/minetest-mods/areas
3. get the warning.
4. insert type = "text", before hud.lua L69
5. No more warning

[grorp]

need protocol version checks

... no? The network format wasn't changed by #14065.

Mods only have to do a server-side feature check (minetest.features.hud_def_type_field)

[cx384]

Ugh, I don't think this is a good idea.
As grorp wrote, you can always check for the features flag.
And this may hide deprecation problems, i.e. if an old mod accidentally used both type and hud_elem_type.
e.g. if it used the result of player:hud_get(id) which always returned a table with type and not hud_elem_type.

[SmallJoker]

... no? The network format wasn't changed by

Oh right. I wonder why I thought that the client version would matter here. Obviously, there is no such need for a transition phase when it's a simple server-side feature check
Sorry for the fuss.