refactor(richtext-lexical)!: restructure index.ts, rename outputSchema to jsonSchema (#16785)

**Breaking:**
- `RichTextAdapter.outputSchema` → `jsonSchema` - matches change in
https://github.com/payloadcms/payload/pull/16783
- Custom-feature `generatedTypes.modifyOutputSchema` ->
`modifyJSONSchema` (and `modifyOutputSchemas` → `modifyJSONSchemas`),
same rationale.

Pure restructuring to make `@payloadcms/richtext-lexical`'s `index.ts`
manageable and to set up a clean `types/` module - landing this first
keeps the upcoming type-safe-lexical PR's diff focused on actual
behavior.

**Moves (no behavior change):**
- `getLexicalHooks` (the four `RichTextHooks`) extracted from `index.ts`
into `hooks.ts`.
- The field-level JSON Schema builder extracted into `types/schema.ts`
as `getFieldToJSONSchema`.
- `types.ts` → `types/index.ts` and `nodeTypes.ts` →
`types/nodeTypes.ts`, establishing a `types/` folder.