EditContext needs a way to inform IMEs of vertical writing modes

When editing vertical text, IMEs may position themselves differently and may layout their own contents differently.
Browsers (or at least Chromium) notify IMEs when the cursor is currently editing vertical text so that they can make these adjustments.

This isn't currently working in EditContext.

One fix would be to check the computed styles of the EditContext's associated element to determine whether to tell the IME it's in vertical versus horizontal writing mode. This is a bit odd though since EditContext doesn't otherwise rely on the associated element's styles, and making changes to those styles in response to EditContext state changes could be an awkward way of managing this.

Another would be to use the computed style at the current cursor's position (but this doesn't work for <canvas>-based EditContext).

I think the right fix is to add a property on EditContext allowing the author to explicitly set whether the current writing mode is horizontal or vertical.

let editContext = new EditContext();
editContext.writingMode; // default is "horizontal"
editContxt.writingMode = "vertical"; // Now we report to IMEs that vertical text is being edited.

// Or just a boolean:
let editContext = new EditContext();
editContext.isVerticalWritingMode; // default is false
editContext.isVerticalWritingMode = true; // Now we report to IMEs that vertical text is being edited.

Something I'm unsure about is whether this should match the options available to CSS writing-mode: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/writing-mode. I'm not sure whether all of those permutations are relevant to IMEs or if the only thing we need is horizontal versus vertical. Even if only horizontal/vertical is needed, maybe there's still a consistency argument that they should be the same?

Start by reviewing the EditContext associated-element definition and the CSS writing-mode options linked in the issue, then compare the horizontal-versus-vertical and full writing-mode proposals. Done means reaching agreement on the API shape and semantics, including the default, and specifying how the selected mode is reported to IMEs.