Ensure language servers only start when a buffer is visible. This helps
with desktop restore speed, as various language servers do not start all
at the same time.
The auto-highlight-symbol package has a nicer interactive editing mode,
but it messes with the visual presentation for selected region and other
things. So let's go back to the less fancy but more reliable
highlight-symbol package.
Also let's properly disable both symbol highlighting packages in golang
and dart, where lsp-mode provides a better and more intelligent symbol
highlighting feature.
