Add an option that will impose `window-size-fixed` on `imenu` buffer.

[DivvyCr]

Based on the discussion of Issue #543.

[DivvyCr]

I would suggest, however, to make this functionality naturally apply with lsp-ui-imenu-window-width. I believe the expected behaviour of that variable imples that the imenu window will be effectively fixed.

Also, on a somewhat unrelated note, I considered adding the no-delete-other-windows property to the imenu window, which basically keeps the window on C-x 1 invocation. Again, I feel that this could be part of the expected behaviour (in the 'persist mode').

[brotzeit]

Can you please add a note https://github.com/emacs-lsp/lsp-ui#lsp-ui-imenu

[brotzeit]

Thanks!