fix(types): allow formStream to be used as a type (#24)

Author: fjc0k

types/formstream.d.ts

@@ -69,4 +69,6 @@ declare const formStream: {
   (): FormStream
 }
 
+interface formStream extends FormStream {}
+
 export = formStream

types/formstream.test-d.ts

@@ -1,8 +1,15 @@
-import formStream from '..'
 import { Readable } from 'stream'
+import formStream from '..'
 
 const fs = formStream()
+const fs2: formStream = formStream()
 const fsNew = new formStream()
+const fsNew2: formStream = new formStream()
+
+// instanceof ok
+if (process instanceof formStream) {
+  process.stream('file', Readable.from('1'), '1.txt')
+}
 
 // @ts-expect-error
 fs.field('id')