explicitly propagate displaysize in sprint

[KristofferC]

Fixes #42649, closes #34721

julia> displaysize(stdout)
(20, 68)

julia> displaysize(IOContext(stdout))
(20, 68)

julia> sprint(; context=stdout) do io
           print(io, displaysize(io))
       end
"(20, 68)"

This also fixes a bug in Pkg where the progress bar has support for shrinking its size when the terminal gets too narrow, but Pkg.precompile uses sprint so it doesn't work there.

It is a bit unclear where to add a test for this...

[timholy]

This seems to fix my concern about bugs. My remaining concern is the scope of this change: if we're changing it in sprint rather than in unwrapcontext, presumably that means we're trying to preserve some of #42649 (comment). But how do we decide which ones should change and which ones should stay as-is? Why is sprint different from other uses? Isn't it kind of the poster child for #42649 (comment)?

I think a better fix for this would be to add a keyword to IOContext(io::IO, context::IO) that controls whether size gets copied, and then pass-through keyword variants of Base functions that call IOContext(io, ctx). That, plus documentation clarifying what happens by default and what can be enabled.

[KristofferC]

That makes sense, I tried to move it to unwrapcontext but running into a bit of a bootstrap problem where displaysize(io::IO) is not defined at that point and the system tries to use this method before it ends up getting defined. And just moving the definition of displaysize(io::IO) up doesn't really work either because it uses ENV which is also not defined. Hm.