Code lenses that show type signatures should be inlay hints instead

Prompted by an issue with `lsp-mode` where they started showing code lenses at the end of the line instead of above the line by default (https://github.com/emacs-lsp/lsp-mode/issues/3749). I made an issue in the spec repo (https://github.com/microsoft/language-server-protocol/issues/1558) to ask whether clients can make any assumptions about where code lenses are displayed.

The answer seems to be _no_, we should not make assumptions about where code lenses are displayed. That means that the way we're using them to display type signatures is fundamentally unreliable. Fortunately, there is a feature that _does_ allow specific placement of something in the buffer: inlay hints.

So I think the "right thing" here is to convert the code lenses that display type signatures into inlay hints.