Move SlashCommands and SlashCommands-test into subdirs (#31979)

* Move SlashCommands into slash-commands

* Move SlashCommands test prep into a function that can be re-used from multiple files

* Move slash command tests into separate files

* Fix super-linear regexes and test some more slash commands

* Move splitAtFirstSpace tests into a separate file

* Test for parseCommandString

* Make parseCommandString able to handle leading whitespace and tabs

* Implement parseCommandString using splitAtFirstSpace

* Extract emoticons slash commands into a separate file and share their code

test/test-utils/test-utils.ts

@@ -351,6 +351,10 @@ export function createTestClient(): MatrixClient {
         },
         search: jest.fn().mockResolvedValue({}),
         processRoomEventsSearch: jest.fn().mockResolvedValue({ highlights: [], results: [] }),
+        invite: jest.fn(),
+        kick: jest.fn(),
+        ban: jest.fn(),
+        sendTextMessage: jest.fn(),
     } as unknown as MatrixClient;
 
     client.reEmitter = new ReEmitter(client);