feat: introduce additional CKEditor theming

[austingreendev]

Description

This PR adds additional CKEditor theming. Its scope is limited to basic styling and doesn't attempt to override any layout, spacing, or advanced functionality at this time.

Detail

Some core areas to focus on are:

• Heading dropdown menu now includes Garden selected/hover/focus styling (not updates to menu positioning yet)
• Disabled heading dropdown now aligns with IconButton styling
• The typearound widget now includes vertical before/after cursors instead of the large the default horizontal implementations.
  • This includes new RTL logic for before/after
• The horizontal rules styling is updated including the margin
• The widget focus styling now mimics the default Garden focus ring. I'm unable to modify the border-radius as this time since inline widgets since that is determined by the individual widget. I will look into this more.
• Override code selected background color shift
• Move inline code to be aligned with bold, italic, underline
• Customize alt+F10 focus states for icon buttons and list focus state.

[m-lai]

@austingreendev Based on your listed changes, all looks kosher to me! The only thing I'm wondering about is whether the chevron in the Heading dropdown menu is vertically center-aligned with the Heading text, it looks slightly off center.

[austingreendev]

Agreed. I'll take a look at the vertical spacing there.

[hzhu]

Had a few questions but LGTM!

[jzempel]

There might need to be a notion of padding if border-radius focus is to be applied to this element.

[austingreendev]

I've updated the horizontal line spacing to match the expected focus-ring size. I'll update any required padding/margin when we handle all of the content spacing in a following PR.

[jzempel]

😍