Visual update for TinyMCE format boundaries

[matveb]

In 4.8 we are introducing a new version of TinyMCE that allows more intuitive positioning of the cursor within inline text elements. By default, TinyMCE doesn't provide much in the way of styling, so we are tweaking the appearance of the boundaries in our custom theme.

props @joen

[azaozz]

This may need more extensive testing as in some cases the styling can interfere with the caret/cursor in contentEditable (things like margin and padding).

Tested in latest Chrome, Firefox, Edge and IE11 and all seems to work well. However will need testing on touch devices, most importantly on devices that have mouse, touch screen, and keyboard like Surface laptops or iPads with Bluetooth keyboards.

[matveb]

In 40783:

TinyMCE: Provide styles for link and code boundaries.

The new version of TinyMCE introduces a cursor-boundary feature for some formatting elements but, by default, it does not provide much in the way of styles. This tweaks their appearance in the WordPress custom skin to match the intended design of the feature.

Props joen.
Fixes #40767.