[TS] Fix relative import paths of generated TypeScript code (#8880)

* Refactor logic that generates import paths in AddImport

* Add new tests to validate relative import path fix

* Generate goldens

* Generate example code

* Format TS generator file

* Revert "Format TS generator file"

This reverts commit 0f0b24aee9.

* Fix merge conflicts

---------

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>

tests/ts/JavaScriptRelativeImportPathTest.js

@@ -0,0 +1,28 @@
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, resolve } from "node:path";
+
+const here = dirname(fileURLToPath(import.meta.url));
+const headerPath = resolve(here, "relative_imports/transit/three/header.ts");
+
+const contents = readFileSync(headerPath, "utf8");
+
+const expectedImports = [
+  "from '../one/info.js';",
+  "from '../two/identity.js';",
+];
+
+for (const expected of expectedImports) {
+  if (!contents.includes(expected)) {
+    throw new Error(`Missing relative import "${expected}" in ${headerPath}`);
+  }
+}
+
+const forbidden = "../transit/";
+if (contents.includes(forbidden)) {
+  throw new Error(
+    `Found unexpected namespace segment in import path within ${headerPath}`
+  );
+}
+
+console.log("JavaScriptRelativeImportPathTest: OK");