Grant the player effectively infinite lives (#1963)

We currently have a partial implementation of lives in the game. The
player starts with 3 lives; they lose a life each time they are
defeated; and when they run out of lives, they are kicked back to the
start of the current challenge.

However, we have no visible indication of the number of lives the player
has remaining. As a result, the game feels broken when the player
unexpectedly loses their progress after being defeated.

In addition, some recent playtesters felt that the game-over behaviour
is unfair, even if it were visible.

For now, set the number of lives to be effectively infinite (the largest
positive integer that Godot can represent), to effectively disable the
feature. This hexadecimal number is 9,223,372,036,854,775,807 in
decimal: 9 quintillion in the short scale.

Helps #1957

Author: wjt

scenes/globals/game_state/game_state.gd

@@ -29,7 +29,7 @@ const GLOBAL_SECTION := "global"
 const GLOBAL_INCORPORATING_THREADS_KEY := "incorporating_threads"
 const COMPLETED_QUESTS_KEY := "completed_quests"
 const LIVES_KEY := "current_lives"
-const MAX_LIVES := 3
+const MAX_LIVES := 0x7fffffffffffffff
 const DEBUG_LIVES := false
 
 ## Scenes to skip from saving.