refactor(plugin-seo): rename non-JSX files from .tsx to .ts (#14418)

### What?

Renamed three non-JSX source files in `@payloadcms/plugin-seo` from
`.tsx` to `.ts` extension and updated `package.json` accordingly.

###  Why?

These files don't contain any JSX syntax, they only export TypeScript
functions and type definitions. Using the `.tsx` extension was incorrect
and inconsistent with all other Payload plugins, which use `.ts` for
their main entry points.

###  How?
- Simple file renames with zero code changes
- Updated package.json exports to reference `./src/index.ts` instead of
`./src/index.tsx`

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: jhb-dev

packages/plugin-seo/package.json

@@ -32,9 +32,9 @@
   "type": "module",
   "exports": {
     ".": {
-      "import": "./src/index.tsx",
-      "types": "./src/index.tsx",
-      "default": "./src/index.tsx"
+      "import": "./src/index.ts",
+      "types": "./src/index.ts",
+      "default": "./src/index.ts"
     },
     "./types": {
       "import": "./src/exports/types.ts",
@@ -52,8 +52,8 @@
       "default": "./src/exports/client.ts"
     }
   },
-  "main": "./src/index.tsx",
-  "types": "./src/index.tsx",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
   "files": [
     "dist"
   ],