feat: Support union underlying type for TS/JS (#7961)

tests/ts/JavaScriptUnionUnderlyingTypeTest.js

@@ -0,0 +1,26 @@
+import assert from 'assert'
+import * as flatbuffers from 'flatbuffers'
+import {UnionUnderlyingType as Test} from './union_underlying_type_test.js'
+
+function main() {
+    let a = new Test.AT();
+    a.a = 1;
+    let b = new Test.BT();
+    b.b = "foo";
+    let c = new Test.CT();
+    c.c = true;
+    let d = new Test.DT();
+    d.testUnionType = Test.ABC.A;
+    d.testUnion = a;
+    d.testVectorOfUnionType = [Test.ABC.A, Test.ABC.B, Test.ABC.C];
+    d.testVectorOfUnion = [a, b, c];
+
+    let fbb = new flatbuffers.Builder();
+    let offset = d.pack(fbb);
+    fbb.finish(offset);
+
+    let unpacked = Test.D.getRootAsD(fbb.dataBuffer()).unpack();
+    assert.equal(JSON.stringify(unpacked), JSON.stringify(d));
+}
+
+main()