fix: add BigInt to type aliases (#1513)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #1482
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/TypeStat/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/TypeStat/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

<!-- Description of what is changed and how the code change does that.
-->

Fixes missing bigint types (finally) 🎉

src/mutations/aliasing/aliases.ts

@@ -6,6 +6,8 @@ import { FileMutationsRequest } from "../../shared/fileMutator.js";
  * Type flags and aliases to check when --strictNullChecks is not enabled.
  */
 const nonStrictTypeFlagAliases = new Map([
+	[ts.TypeFlags.BigInt, "bigint"],
+	[ts.TypeFlags.BigIntLiteral, "bigint"],
 	[ts.TypeFlags.Boolean, "boolean"],
 	[ts.TypeFlags.BooleanLiteral, "boolean"],
 	[ts.TypeFlags.Number, "number"],

test/cases/fixes/incompleteTypes/returnTypes/expected.ts

@@ -107,4 +107,16 @@
 	async function returnPromise(): Promise<string> {
 		return Promise.resolve("");
 	}
+
+	const returnsBigInt = (): string | bigint => {
+		return BigInt("123");
+	};
+
+	const returnsBigintOrNumber = (): number | bigint => {
+		return 123n;
+	};
+
+	const returnsNumberAndBigint = (): bigint | number => {
+		return 123;
+	};
 })();

test/cases/fixes/incompleteTypes/returnTypes/original.ts

@@ -107,4 +107,16 @@
 	async function returnPromise(): Promise<string> {
 		return Promise.resolve("");
 	}
+
+	const returnsBigInt = (): string => {
+		return BigInt("123");
+	};
+
+	const returnsBigintOrNumber = (): number => {
+		return 123n;
+	};
+
+	const returnsNumberAndBigint = (): bigint => {
+		return 123;
+	};
 })();