Add console.clear from the Console API

Can't keep this in the tests, as it (successfully) clears the test results

src/console_ffi.mjs

···
 export function assert(assertion, value) {
   console.assert(assertion, value);
 }
+
+export function clear() {
+  console.clear();
+}

src/plinth/javascript/console.gleam

···
 /// If the assertion is true, nothing happens.
 @external(javascript, "../../console_ffi.mjs", "assert")
 pub fn assert_(assertion: Bool, value: a) -> Nil
+
+/// clears the console if the console allows it. A graphical console,
+/// like those running on browsers, will allow it; a console displaying
+/// on the terminal, like the one running on Node, will not support it,
+/// and will have no effect (and no error).
+@external(javascript, "../../console_ffi.mjs", "clear")
+pub fn clear() -> Nil