Enable eslint rule 'no-fallthrough'

Enable recommended rule 'no-fallthrough' [1] and manually fix violations.

[1] https://eslint.org/docs/latest/rules/no-fallthrough

Ref: #8068
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

binaries/data/mods/public/gui/session/chat/ChatMessageFormatSimulation.js

@@ -188,7 +188,7 @@ ChatMessageFormatSimulation.flare = class
 		case "observer":
 			if (!g_IsObserver)
 				return "";
-
+			break;
 		default:
 			break;
 		}

eslint.config.mjs

@@ -49,6 +49,7 @@ const configEslintRecommended = {
 		"no-empty-static-block": "warn",
 		"no-ex-assign": "warn",
 		"no-extra-boolean-cast": "warn",
+		"no-fallthrough": "warn",
 		"no-func-assign": "warn",
 		"no-global-assign": "warn",
 		"no-import-assign": "warn",