Eldoc support

This adds support for eldoc and documentation popup for corfu-doc/company-quickhelp and such.

Eldoc displays function signatures and variable documentation in the echo area. For function it highlights argument based on point position:

Additionally, eldoc can distinguish function vs method calls:

Variable documentation can be displayed in the echo line if the point is not on a function call:

The documentation popup for corfu-doc looks like this:

It supports markdown rendering via optional markdown-mode.