Started basic playground for testing lexical as a new WYSIWYG editor.
Moved out tinymce to be under wysiwyg-tinymce instead so lexical is the
default, but TinyMce code remains.
Markdown-related code was growing, representing half of app.js main
bundle code while only being needed in one view/scenario.
This extracts markdown related code to its own built file.
Related to #4858
Avoided updating markdown-it package to 14 for now since it would cause
bundle size to inflate. Don't think ESBuild is properly tree shaking
"entities" sub package which inflates size.
Allows us to use NodeJS code for file/directory locating to not be
shell/os specific, while also also reducing duplicated complexity within
packages.json file.
Related to #3323
