Write put sideline text inside the margin when using olivetti-mode

[plcplc]

Olivetti-mode and Writeroom-mode reduce the buffer body width to center text on the screen, leaving ample empty space in the margins of the window.

Example:

lsp-ui-sideline lets us view things such as docs and diagnostics by the end of the line.

They do not use the available margin space, but remain within the restricted body width.

It seems like a good opportunity to be able to use that margin space.