Add Rich Editor block icons, grid layout, search, and sticky controls

[imliam]

Description

In an application with a lot of custom blocks in the rich editor, the current UI for the block panel gets a bit unwieldy. This PR pulls together a few core improvements to the block panel:

• Custom blocks can define their own icons
• The custom blocks panel can have a "grid" layout which looks neater with many different blocks
• The custom blocks panel can have a search bar, to make it easier for users to find the block they want when the list is exhaustive
• The toolbar and panels can be marked as sticky, so when scrolling down the page on an editor with lots of content, all of the controls are still at hand

These options can all be set independently of one another on the rich editor; combined hopefully making a nicer experience for large/complex editors.

RichEditor::make('content')
    ->stickyToolbar()
    ->stickyPanels()
    ->customBlocks(...)
    ->customBlocksGrid()
    ->searchableCustomBlocks();

Visual changes

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.