feat: add a Flow-to-TypeScript migration (#1)

* Revert "wow"

This reverts commit 7d07f40.

* back to basics

* does not work

* match snapshot

.grit/patterns/FlowToTypeScript.md

@@ -0,0 +1,99 @@
+# Convert Flow to TypeScript
+
+Converts Flow type annotations to TypeScript type annotations on a best-effort basis.
+
+```grit
+Program(and {
+  [
+    maybe bubble or  {
+      ImportDeclaration(leadingComments = [CommentBlock($c), ...]),
+      ExportDeclaration(leadingComments = [CommentBlock($c), ...])
+    } as $node => raw("/*" + $c + "*/\n" + unparse($node))
+    some bubble or { ImportDeclaration(), ExportDeclaration() } as $node => raw(unparse($node)) 
+  ]
+  maybe contains bubble TypeAnnotation() as $node => raw(unparse($node))
+})
+```
+
+## Transform comment-style type annotations
+
+```js
+//@flow
+/*:: import type { Foo, Sam } from '../types';*/
+/*:: import type { Dog } from './animals';*/
+/*:: export type DogBreed = {
+  name: string,
+} */
+const animal = 'dog';
+
+function checkDog(dog/*: Dog */)/*: string */ {
+  return dog.name;
+};
+
+function multiLine({
+    foo,
+    bar
+}/*: {
+  foo: string,
+  bar: string
+  } */) {
+  console.log(foo);
+}
+
+const checkAnimalBreed = async (
+  { breed, dog } /*: {
+    breed: DogBreed,
+    dog: Dog,
+  } */,
+)/*: boolean */ => {
+    return dog.breed === breed.name;
+};
+
+const checkBoolean = async ()/*: boolean */ => {
+    return false;
+};
+
+export default checkAnimalBreed;
+```
+
+```js
+//@flow
+import type { Foo, Sam } from '../types';
+import type { Dog } from './animals';
+export type DogBreed = {
+  name: string,
+}
+const animal = 'dog';
+
+function checkDog(dog: Dog): string {
+  return dog.name;
+}
+
+function multiLine({
+  foo,
+  bar
+}: {
+  foo: string,
+  bar: string
+  }) {
+  console.log(foo);
+}
+
+const checkAnimalBreed = async (
+  {
+    breed,
+    dog
+  }: {
+      breed: DogBreed,
+      dog: Dog,
+    }
+): boolean => {
+    return dog.breed === breed.name;
+};
+
+const checkBoolean = async (): boolean => {
+    return false;
+};
+
+export default checkAnimalBreed;
+```