Updates paths for Node16 module resolution.

Node16 module resolution [does not support "directory modules"](https://www.typescriptlang.org/docs/handbook/modules/reference.html#supported-features), yet the `paths` option was linking to a directory. This updates the `paths` to link directly to the associated file or default to `index.mjs`.

This isn't an ideal solution as it does not respect `exports` and effectively requires that any exported subpath link to an internal file at exact same path. [TS recommendation](https://www.typescriptlang.org/docs/handbook/modules/reference.html#paths-should-not-point-to-monorepo-packages-or-node_modules-packages) is to avoid `paths` for monorepo resolution for this reason and instead rely on `pnpm` linking. I made an attempt at this and found that it works for `@rules_prerender/declarative_shadow_dom` and `@rules_prerender/preact` pretty well, since they link against their deps via the `workspace:` protocol already. However it doesn't work for the rest of the workspace because they rely on the `npm_link_all_packages` target and if the `package.json` depends on `rules_prerender` or `@rules_prerender/*` packages, then it will duplicate the existing `//:node_modules/rules_prerender`-style targets. However we can't get rid of the existing ones since those build from real source.

For now we'll just a less-than-optimal `paths` option, however we should explore in the future how to drop it entirely and rely on `pnpm` linking to solve this problem.

packages/rules_prerender/package.json

@@ -2,5 +2,6 @@
     "name": "rules_prerender",
     "version": "0.0.0-PLACEHOLDER",
     "type": "module",
-    "main": "./index.mjs"
+    "main": "./index.mjs",
+    "types": "./index.d.mts"
 }

tsconfig.json

@@ -39,8 +39,12 @@
     "moduleResolution": "Node16",             /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
     "baseUrl": "./",                          /* Base directory to resolve non-absolute module names. */
     "paths": {                                /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
-      "rules_prerender": ["packages/rules_prerender"], // Map `rules_prerender` module to its implementation.
-      "@rules_prerender/*": ["packages/*"],
+      "rules_prerender": ["packages/rules_prerender/index.mts"],
+      "rules_prerender/*": ["packages/rules_prerender/*"],
+      "@rules_prerender/declarative_shadow_dom": ["packages/declarative_shadow_dom/index.mts"],
+      "@rules_prerender/declarative_shadow_dom/*": ["packages/declarative_shadow_dom/*"],
+      "@rules_prerender/preact": ["packages/preact/index.mts"],
+      "@rules_prerender/preact/*": ["packages/preact/*"],
     },
     // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
     // "typeRoots": [],                       /* List of folders to include type definitions from. */