Fix potential security issue fontifying lsp doc

Fix potential security issue fontifying lsp doc

Previously, a server could mistankely or maliciously call *-mode
functions by in the response to a completion or hover request,
specifically in the :documentation field of the response.

Although there are plenty of similar avenues of attack in Emacs, it's
probably a good idea not to let LSP servers decide which functions to
call in an Emacs session running Eglot.

* eglot.el (eglot--format-markup): Call major-mode to fontify
buffer, not some dynamically constructed function name.
(eglot-completion-at-point): Ensure eglot--format-markup runs in
source buffer.

GitHub-reference: fix https://github.com/joaotavora/eglot/issues/154