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>

ts/flexbuffers.ts

@@ -1,4 +1,3 @@
-/* eslint-disable @typescript-eslint/no-namespace */
 import {Builder} from './flexbuffers/builder.js';
 import {toReference} from './flexbuffers/reference.js';
 export {toReference} from './flexbuffers/reference.js';