[lexical-playground] [minor] Feature: turn single-file plugins `plugins/*Plugin/index.tsx` into `plugins/*Plugin.tsx`

Similar to https://github.com/facebook/lexical/blob/main/packages/lexical-playground/src/plugins/TablePlugin.tsx

The import statements like `import LinkPlugin from './plugins/LinkPlugin'` do work with both single-file and directory style, but the non-`index.tsx`-named plugin files are a tiny bit more convenient to manage in custom user codebases which copy/sync these plugins code from Playground.

Out of 50 plugin dirs currently, only 13 plugins are multi-file, the remaining 37 are single-file `index.tsx` ones