support markdown editor

[ziming]

This PR support markdown editor.

It achieves this by adding a new config setting editor. by default it is set to rich but user can change to markdown

Here is a screenshot of how it looks like when editor is markdown mode

Some notes:

• The delete icon is not shown because my policy class set delete to false
• From what i see in filament docs, toolbar buttons are pretty much the same for markdown & rich text editor

https://filamentphp.com/docs/3.x/forms/fields/markdown-editor#customizing-the-toolbar-buttons

• I removed ->extraInputAttributes(['style' => 'min-height: 6rem']) from the MarkdownEditor because MarkdownEditor does not support the ->extraInputAttributes() method