Parser: allow 'options' to explicitly accept undefined

Continuation of graphql#3670
Basically allow to have code like this:
```js
parse(document, GraphQLServerConfig.parserOptions)
```
to pass TS check with 'exactOptionalPropertyTypes'

src/language/parser.ts

@@ -129,7 +129,7 @@ export interface ParseOptions {
  */
 export function parse(
   source: string | Source,
-  options?: ParseOptions,
+  options?: ParseOptions | undefined,
 ): DocumentNode {
   const parser = new Parser(source, options);
   return parser.parseDocument();
@@ -147,7 +147,7 @@ export function parse(
  */
 export function parseValue(
   source: string | Source,
-  options?: ParseOptions,
+  options?: ParseOptions | undefined,
 ): ValueNode {
   const parser = new Parser(source, options);
   parser.expectToken(TokenKind.SOF);
@@ -162,7 +162,7 @@ export function parseValue(
  */
 export function parseConstValue(
   source: string | Source,
-  options?: ParseOptions,
+  options?: ParseOptions | undefined,
 ): ConstValueNode {
   const parser = new Parser(source, options);
   parser.expectToken(TokenKind.SOF);
@@ -183,7 +183,7 @@ export function parseConstValue(
  */
 export function parseType(
   source: string | Source,
-  options?: ParseOptions,
+  options?: ParseOptions | undefined,
 ): TypeNode {
   const parser = new Parser(source, options);
   parser.expectToken(TokenKind.SOF);
@@ -204,10 +204,10 @@ export function parseType(
  * @internal
  */
 export class Parser {
-  protected _options: Maybe<ParseOptions>;
+  protected _options: ParseOptions;
   protected _lexer: Lexer;
 
-  constructor(source: string | Source, options?: ParseOptions) {
+  constructor(source: string | Source, options: ParseOptions = {}) {
     const sourceObj = isSource(source) ? source : new Source(source);
 
     this._lexer = new Lexer(sourceObj);
@@ -472,7 +472,7 @@ export class Parser {
   parseNullabilityAssertion(): NullabilityAssertionNode | undefined {
     // Note: Client Controlled Nullability is experimental and may be changed or
     // removed in the future.
-    if (this._options?.experimentalClientControlledNullability !== true) {
+    if (this._options.experimentalClientControlledNullability !== true) {
       return undefined;
     }
 
@@ -575,7 +575,7 @@ export class Parser {
     // Legacy support for defining variables within fragments changes
     // the grammar of FragmentDefinition:
     //   - fragment FragmentName VariableDefinitions? on TypeCondition Directives? SelectionSet
-    if (this._options?.allowLegacyFragmentVariables === true) {
+    if (this._options.allowLegacyFragmentVariables === true) {
       return this.node<FragmentDefinitionNode>(start, {
         kind: Kind.FRAGMENT_DEFINITION,
         name: this.parseFragmentName(),
@@ -1455,7 +1455,7 @@ export class Parser {
     startToken: Token,
     node: T,
   ): T {
-    if (this._options?.noLocation !== true) {
+    if (this._options.noLocation !== true) {
       node.loc = new Location(
         startToken,
         this._lexer.lastToken,