Implement nullability operators in query language

The ["Nullability RFC" for GraphQL](graphql/graphql-wg#694)
allows fields to individually be marked as optional or required in a query by the
client-side. ([See Strawman Proposal](graphql/graphql-spec#867))

If a field is marked as optional then it's allowed to be missing and `null`, which
can control where missing values cascade to:

```graphql
query {
  me {
    name?
  }
}
```

If a field is marked as required it may never be allowed to become `null` and must
cascade if it otherwise would have been set to `null`:

```graphql
query {
  me {
    name!
  }
}
```

Author: kitten

alias/language/__tests__/printer.test.js

@@ -54,6 +54,14 @@ describe('Printer: Query document', () => {
         name
       }
     `);
+
+    const queryWithNullabilityFields = parse('query { id?, name! }');
+    expect(print(queryWithNullabilityFields)).toBe(dedent`
+      {
+        id?
+        name!
+      }
+    `);
   });
 
   it('prints query with variable directives', () => {

alias/language/parser.mjs

@@ -147,12 +147,20 @@ const directives = match()`
   ${directive}*
 `;
 
+const nullability = match(null, (x) => {
+  return x[0] === '?' ? 'optional' : 'required';
+})`
+  :${ignored}?
+  ${/[?!]/}
+`;
+
 const field = match(Kind.FIELD, (x) => {
   let i = 0;
   return {
     kind: x.tag,
     alias: x[1].kind === Kind.NAME ? x[i++] : undefined,
     name: x[i++],
+    required: typeof x[i] === 'string' ? x[i++] : 'unset',
     arguments: x[i++],
     directives: x[i++],
     selectionSet: x[i++],
@@ -164,6 +172,7 @@ const field = match(Kind.FIELD, (x) => {
     (?: ${ignored}? ${':'} ${ignored}?)
     ${name}
   )?
+  ${nullability}?
   ${args}
   ${directives}
   ${() => selectionSet}?

alias/language/printer.mjs

@@ -36,11 +36,15 @@ export function print(node) {
       );
 
     case 'Field':
+      let prefix = wrap('', print(node.alias), ': ') + print(node.name);
+      if (node.required === 'optional') {
+        prefix += '?';
+      } else if (node.required === 'required') {
+        prefix += '!';
+      }
       return join(
         [
-          wrap('', print(node.alias), ': ') +
-            print(node.name) +
-            wrap('(', join(print(node.arguments), ', '), ')'),
+          prefix + wrap('(', join(print(node.arguments), ', '), ')'),
           join(print(node.directives), ' '),
           print(node.selectionSet),
         ],