Prettier 2: Electric Boogaloo (#6113)

# Description of Changes
When I added Prettier formatting in #6052, my aim was to use just the
default settings in Prettier. Turns out, Prettier looks _really hard_
for any config files if it's not explicitly given one, which means that
if a developer has some sort of Prettier config file lying around on
their system, Prettier might find it and use it. Also, Prettier changes
its defaults based on stuff in `.editorconfig` without any good way of
disabling that behaviour explicitly in its config file.

To solve both of these issues, I've introduced a `.prettierrc` file
which sets Prettier's defaults explicitly, and then reformatted all our
code _again_ in Prettier's actual default settings. This should achieve
the aim of #6052 and remove the possibility for it breaking on different
dev computers.

frontend/src/core/testing/serverExperienceSimulations.ts

@@ -165,10 +165,14 @@ function getActiveScenario(): SimulationScenario | null {
   }
   const scenario = SIMULATION_SCENARIOS[SIMULATION_INDEX];
   if (!scenario) {
-    console.warn("[Simulation] SIMULATION_INDEX out of range, using live backend.");
+    console.warn(
+      "[Simulation] SIMULATION_INDEX out of range, using live backend.",
+    );
     return null;
   }
-  console.warn(`[Simulation] Using scenario #${SIMULATION_INDEX} (${scenario.label}).`);
+  console.warn(
+    `[Simulation] Using scenario #${SIMULATION_INDEX} (${scenario.label}).`,
+  );
   return scenario;
 }