Implement editor.uploadFile syscall (#570)

* Implement editor.attachFile syscall

* Refactor attachFile to uploadFile

returns a promise with an UploadFile now

* Fix style

* Reject promise with errors

* Another code style fix

---------

Co-authored-by: prcrst <p-github@prcr.st>

plug-api/types.ts

@@ -140,3 +140,8 @@ export type LintDiagnostic = {
   severity: "error" | "warning" | "info" | "hint";
   message: string;
 };
+
+export type UploadFile = {
+  name: string;
+  content: Uint8Array;
+}