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>

include/flatbuffers/idl.h

@@ -734,6 +734,7 @@ struct IDLOptions {
   bool python_gen_numpy;
 
   bool ts_omit_entrypoint;
+  bool ts_undefined_for_optionals;
   ProtoIdGapAction proto_id_gap_action;
 
   // Possible options for the more general generator below.
@@ -860,6 +861,7 @@ struct IDLOptions {
         python_typing(false),
         python_gen_numpy(true),
         ts_omit_entrypoint(false),
+        ts_undefined_for_optionals(false),
         proto_id_gap_action(ProtoIdGapAction::WARNING),
         mini_reflect(IDLOptions::kNone),
         require_explicit_ids(false),