Fix yarn lint:types when matrix-js-sdk is not yarn linked. (#30612)

* Add missing dependencies on `@types` packages

Because we import the typescript source from matrix-js-sdk rather than the
`.d.ts` files, `tsc` ends up type-checking the js-sdk source. That means that
we need to have the `@types` packages that js-sdk needs.

* Add missing type definitions for `setInterval` and `setTimeout`

Our source assumes that `setTimeout` returns a number, not a
`Timeout`. If we `yarn link` js-sdk, then (somehow) we end up using the
definitions from there, but it's not really correct.

* Configure knip to ignore new deps

knip.ts

@@ -42,6 +42,13 @@ export default {
         "util",
         // Embedded into webapp
         "@element-hq/element-call-embedded",
+
+        // Used by matrix-js-sdk, which means we have to include them as a
+        // dependency so that // we can run `tsc` (since we import the typescript
+        // source of js-sdk, rather than the transpiled and annotated JS like you
+        // would with a normal library).
+        "@types/content-type",
+        "@types/sdp-transform",
     ],
     ignoreBinaries: [
         // Used in scripts & workflows