Closes #4448

Ligatures were disabled since some editors didn’t allow placing the cursor inside of them.

“Contextual alternatives” on the other hand still allow this, therefore this commit enables them to support e.g. Fira Code’s main gimmick.

Since the input object might try to read from that on focusing

Issue #4439

It sometimes produces invalid output.

Closes #4410

Closes #4412

Issue #4426

Issue #4417

Closes #4437

Closes #4432

Issue #4413

Adds an option to keep the active line highlighted even when text
inside the line is selected.

so other potential emacs-type modules can use

Closes #4419

Add template/variable definitions, checking, types, additional
keywords and indentation fixes.

Closes #4401

Issue #4307

As per CommonMark (conflicting with markdown.pl, but never
mind markdown.pl)

Closes #4395

when accessing `document.activeElement` from inside an iframe.

So that the code doesn't get confused when backspacing or
deleting across a line.

This is still flaky. Ideally we'd capture backspace as a key event,
but Android Chrome makes that impossible.

Issue #4307

Rather than on read

And do so only after a delay, so that subsequent input
events get a chance to fire.