Restore and fix macOS dark text selection cursor

[kdrag0n]

This PR fixes #96946 by restoring the old cursor for all macOS versions, because even on macOS Catalina, the old special cursor is easier on the eyes when there's a dark background. The continued use of a special cursor can be seen in other apps as well, such as iTerm 2.

The cursor was originally removed in a915df1 to fix #94144, but removing it entirely isn't necessary. This PR fixes the inconsistency issues exhibited in the old implementation of the special cursor by adding a shared mouse-text class to all editor-related elements that need a text selection cursor in their entire bounding box and applying the special cursor to that class rather than just .monaco-editor .view-lines.

The custom dark cursor is now consistent in the following areas:

• Text caret (which was the original issue behind Blinking cursor outline when selecting with mouse #94144)
• Editor text view
• Diff editor
• GitHub review comments and "Reply" field
• Debug console REPL

[msftclas]

All CLA requirements met.

[alexdima]

I moved some things around, to get this working also for the standalone editor. The changes in the editor LGTM.

cc @isidorn for a small change to adopt in the repl
cc @rebornix for the change to adopt in the comments thread

[isidorn]

This line should not be removed. This controls the looks of the cursor inside the debug console tree and is unrelated to this PR.

[alexdima]

@isidorn That is the entire point of this PR, to change how the mouse cursor looks like on mac, but it goes even outside of the editor and adopts the custom cursor that the editor had also in the repl and in the comments thread.

[isidorn]

Aha, I see that the class is set on the repl-tree.
I misunderstood the PR that it changes cursor only in the editor and all the editor instances like the repl input.
So I take my original comment back.

[isidorn]

I have added one comment which I think should be tackled. Otherwise the change in repl.ts looks good.
I do not understand why is a change in textAreaHandler needed? This textArea is not visible.

[alexdima]

The textarea can become visible when doing input composition, like e.g. typing Japanese text.

[isidorn]

Good point, thanks.

[kdrag0n]

I wasn't actually thinking of IMEs when I added the text cursor class to the textarea — it was because it's one of the elements causing #94144 (the other element being the cursor div). It's not necessarily visible, but there's still a 1 px mouse hover area that it occupies, so the cursor changes when moving over the textarea.

[alexdima]

@rebornix I have tried this locally and the changes in the comment thread look good to me, I think it makes sense to have the same I cursor everywhere, in the editor, and in the comment body/title/reply button.

I will optimistically merge, since this looks good and people are complaining (e.g. #97550) and I'd like to have a fix out in Insiders.

[rebornix]

@alexdima thanks for the ping, it looks good to me!