fix: marshal Undefined and SafeString DOM values in Lua

`domValue_push` handled `Null`, `Boolean`, `Integer`, `String`, `Array`,
and `Object`, and aborted via `MRDOCS_UNREACHABLE` for any other kind.
Reading a field whose value is `Undefined` or `SafeString` therefore
crashed a Lua script.

`Undefined` is common: a symbol with no name (the global namespace, for
instance) has an `Undefined` name, so a Lua script that reads
`symbol.name` aborted the build.

So, map `Undefined` to `nil`, as `Null` already is, and push a
`SafeString` as its bytes, the way `String` is handled. This matches the
JavaScript bridge.

Author: gennaroprota

src/lib/Support/Lua.cpp

@@ -643,11 +643,20 @@ domValue_push(
     {
     case dom::Kind::Null:
         return lua_pushnil(A);
+    case dom::Kind::Undefined:
+        // Lua has a single nullary value, so a missing field maps to
+        // `nil` just as `Null` does. A read of an absent field (for
+        // example the global namespace's name) yields `Undefined` and
+        // must not abort.
+        return lua_pushnil(A);
     case dom::Kind::Boolean:
         return lua_pushboolean(A, value.getBool());
     case dom::Kind::Integer:
         return lua_pushnumber(A, value.getInteger());
     case dom::Kind::String:
+    case dom::Kind::SafeString:
+        // A `SafeString` is a string already marked safe for an output
+        // format; to a Lua script it is just its bytes.
         return luaM_pushstring(A, value.getString());
     case dom::Kind::Array:
         return domArray_push(A, value.getArray());