Move 'abort' out of builtins and make it overridable

Now defaults to 'env.abort' as imported in lib/env.ts, but can be overridden through '--use abort=someThingElseOrNothingAtAll'

std/assembly/env.ts

@@ -0,0 +1,7 @@
+/** Environment abort function called where assertions evaluate to false / on throw. */
+declare function abort(
+  message?: string | null,
+  fileName?: string | null,
+  lineNumber?: u32,
+  columnNumber?: u32
+): void;