LibJS: Parse slashes after reserved identifiers correctly

Previously we were unable to parse code like `yield/2` because `/2`
was parsed as a regex. At the same time `for (a in / b/)` was parsed
as a division.

This is solved by defaulting to division in the lexer, but calling
`force_slash_as_regex()` from the parser whenever an IdentifierName
is parsed as a ReservedWord.

Userland/Libraries/LibJS/Tests/modules/default-regexp-export.mjs

@@ -0,0 +1,3 @@
+export default /foo/;
+
+export let passed = true;