Handle memory errors occurring in wasm scanners

* In WASM, use a custom, simple malloc implementation that lets us
  expicitly reset the heap with a new start location.
* When a WASM call traps or errors, propagate that as a parse failure.
* Reset the WASM heap after every parse.

Co-authored-by: Conrad <conrad@zed.dev>

cli/src/tests/helpers/allocations.rs

@@ -81,7 +81,6 @@ fn record_alloc(ptr: *mut c_void) {
 }
 
 fn record_dealloc(ptr: *mut c_void) {
-    assert!(!ptr.is_null(), "Zero pointer deallocation!");
     RECORDER.with(|recorder| {
         if recorder.enabled.load(SeqCst) {
             recorder