bug#61412: Inlay activation

[Chinmay Dalal]

> Yes.  I haven't  looked at your code, but my expectation is for inlay hints to
> activate either on a timer after each change, or by the explicit request of
> the user via a keybinding.

My code updates inlay hints on every save (`after-save-hook`).

Regarding keybinding, on the github discussion someone suggested that
vscode shows them while a key combination is held and hides them when
they are released, but I was told emacs can't do this.

Regarding timer, personally I find it annoying when lsp-mode starts
showing overlays when I'm not done typing a line, which is why I chose
after-save-hook. For me it would be best if they were updated every time
I pressed enter, but I don't know how to implement that.