chore(emacs): minor adjustments to synthax hightlight and input mode

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(server): add FINDG and FINDP documentation, closes #144

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs/lean-cmd): extend info to have column-number
feat(emacs/lean-info): show extra-type info for expr in parens

Close #135
checkpoint for imenu