Partner commanders: Partner, Partner—<variant>, Partner with, Background, Doctor

A Commander/Brawl deck can now have two commanders when their abilities
allow it. Partner abilities are parsed from oracle text (Scryfall's keyword
list files every "Partner—Survivors"-style variant under plain "Partner"),
and a second search box offers only cards that can pair with the commander.
The legality check accepts a valid pair and checks color identity against
both; suggestions and decklist import/export handle both commanders.

Closes #4

src-tauri/src/lib.rs

@@ -103,6 +103,8 @@ pub fn run() {
             commands::duplicate_deck,
             commands::set_deck_format,
             commands::set_deck_commander,
+            commands::set_deck_commanders,
+            commands::search_partners,
             commands::set_deck_card,
             commands::deck_contents,
             commands::suggest_cards,