Support tab-separated deck imports (#4222)

Third-party deck tools (e.g. Delver Lens) export decks as tab-separated rows
such as "2	Card Name	Set Name". Card names never contain tab characters,
so a leading numeric column marks a tabular record: the first two columns are read
as the quantity and card name, and any further columns are discarded as
printer/set metadata.

- deck_list: parse tab-delimited rows before whitespace collapsing
- tests: cover tab-separated mainboard and sideboard imports
- docs: document the accepted plain-text import format (fills the TODO)

tests/loading_from_clipboard/loading_from_clipboard_test.cpp

@@ -74,6 +74,20 @@ TEST(LoadingFromClipboardTest, WeirdWhitespaceIsIgnored)
     testDeck(clipboard, result);
 }
 
+TEST(LoadingFromClipboardTest, TabSeparatedImport)
+{
+    QString clipboard("2\tMystic Snake\tStreets of New Capenna\n"
+                      "3x\tCounterspell\tNPH\n"
+                      "\t2\tSol Ring\tFMB\n"
+                      "1\tForest\n"
+                      "\n"
+                      "2x\tDoom Blade\tM11\n");
+
+    Result result("", "", {{"Mystic Snake", 2}, {"Counterspell", 3}, {"Sol Ring", 2}, {"Forest", 1}},
+                  {{"Doom Blade", 2}});
+    testDeck(clipboard, result);
+}
+
 TEST(LoadingFromClipboardTest, RemoveBlankEntriesFromBeginningAndEnd)
 {
     QString clipboard("\n"