Add --ts-undefined-for-optionals command line option (#8861)

* Add --ts-undefined-for-optionals command line option

# Details

- Fixes #7656
- Added a new `--ts-undefined-for-optionals` command line option for `flatc`.
- If enabled, generated TypeScript code uses `undefined` for optional fields rather than `null`.

* Also add TS generated test files

* Run `sh scripts/clang-format-git.sh`

* also add tests/ts/lalala-options.ts to the repo

* move new tests to tests/ts/optional_values dir

* add tests/ts/optional_values/optional_values_generated.cjs to the repo

* reuse existing optional_scalars.fbs and add new test

* add comma

* sh scripts/clang-format-git.sh

* remove comma

* sh scripts/clang-format-git.sh

* trying things

* sh scripts/clang-format-git.sh

* done

* address feedback

* sh scripts/clang-format-git.sh

* run `sh scripts/clang-format-git.sh`

* remove uneeded `eslint-disable @typescript-eslint/no-namespace` line

---------

Co-authored-by: José Luis Millán <jmillan@aliax.net>

tests/ts/JavaScriptUndefinedForOptionals.js

@@ -0,0 +1,60 @@
+import assert from 'assert'
+import * as flatbuffers from 'flatbuffers'
+
+import optional_scalars from './ts-undefined-for-optionals/optional_scalars_generated.cjs'
+
+const { ScalarStuff, ScalarStuffT } = optional_scalars.optional_scalars;
+
+function testScalarStuffBuf(scalarStuff) {
+  assert.strictEqual(scalarStuff.justI8(), -1);
+  assert.strictEqual(scalarStuff.maybeI8(), undefined);
+  assert.strictEqual(scalarStuff.defaultI8(), 42);
+  assert.strictEqual(scalarStuff.justU8(), 1);
+  assert.strictEqual(scalarStuff.maybeU8(), undefined);
+  assert.strictEqual(scalarStuff.defaultU8(), 42);
+}
+
+function testScalarStuffUnpack(scalarStuff) {
+  assert.strictEqual(scalarStuff.justI8, -1);
+  assert.strictEqual(scalarStuff.maybeI8, undefined);
+  assert.strictEqual(scalarStuff.defaultI8, 42);
+  assert.strictEqual(scalarStuff.justU8, 1);
+  assert.strictEqual(scalarStuff.maybeU8, undefined);
+  assert.strictEqual(scalarStuff.defaultU8, 42);
+}
+
+function createScalarStuff(fbb) {
+  ScalarStuff.startScalarStuff(fbb);
+  ScalarStuff.addJustI8(fbb, -1);
+  ScalarStuff.addJustU8(fbb, 1);
+  var offset = ScalarStuff.endScalarStuff(fbb);
+  ScalarStuff.finishScalarStuffBuffer(fbb, offset);
+}
+
+function main() {
+  var fbb = new flatbuffers.Builder();
+
+  createScalarStuff(fbb);
+
+  var buf = new flatbuffers.ByteBuffer(fbb.asUint8Array());
+  var scalarStuff = ScalarStuff.getRootAsScalarStuff(buf);
+
+  testScalarStuffBuf(scalarStuff);
+
+  testScalarStuffUnpack(scalarStuff.unpack());
+
+  var scalarStuff_to = new ScalarStuffT();
+  scalarStuff.unpackTo(scalarStuff_to);
+
+  testScalarStuffUnpack(scalarStuff_to);
+
+  fbb.clear();
+  ScalarStuff.finishScalarStuffBuffer(fbb, scalarStuff_to.pack(fbb));
+  var unpackBuf = new flatbuffers.ByteBuffer(fbb.asUint8Array());
+
+  testScalarStuffBuf(ScalarStuff.getRootAsScalarStuff(unpackBuf));
+
+  console.log('FlatBuffers --ts-undefined-for-optionals test: completed successfully');
+}
+
+main();