Log through a GameLog autoload instead of print()

GameLog.debug() prints only in debug builds, and GameLog.warning() reports
problems such as unexpected server data or failed downloads.

tests/unit/test_game_log.gd

@@ -0,0 +1,23 @@
+extends GdUnitTestSuite
+## GameLog joins its arguments like print() does.
+
+
+func test_joins_parts_without_spaces_like_print() -> void:
+	assert_str(GameLog._join(["Song ", 3, " of ", 10.5])).is_equal("Song 3 of 10.5")
+
+
+func test_joins_any_kind_of_value() -> void:
+	assert_str(GameLog._join([null, true, [1, 2], {"a": 1}])) \
+			.is_equal("<null>true[1, 2]{ \"a\": 1 }")
+
+
+func test_no_parts_gives_an_empty_line() -> void:
+	assert_str(GameLog._join([])).is_empty()
+
+
+func test_debug_takes_any_number_of_arguments() -> void:
+	GameLog.debug()
+	GameLog.debug("one")
+	GameLog.debug("one", 2, Vector2.ONE)
+	# Reaching this line means none of the calls failed
+	assert_bool(true).is_true()

tests/unit/test_game_log.gd.uid

@@ -0,0 +1 @@
+uid://delnqk8yt0hes