Make a default TailwindCSS theme

[coltisor]

Finish the https://github.com/coltisor/lexical-toolkit/blob/main/packages/playground/src/components/RichTextEditor/theme.ts theme implementation (replace al "RTE_" classes with TailwindCSS classes, and expose this file it as a default theme in the package.

This is going to be very useful to jumpstart the Lexical development experience.