The two new hooks allow to execute user code after certain events.
They are called with the language server table as first argument.

 * initialized_hook is called after the initialized notification
   was sent.
 * file_open_hook is called after sending a textDocument/didOpen
   notification.
successfully initialized and when a new file is opened by the editor.

Including methods in the language server table allows them to be used
outside of vis-lspc.

Additionally, I think this makes more sense to use them as methods
with the ls:method(...) syntax instead of calling them as ls_*(ls, ...).

This allows us to support opening a single file in multiple
language server instances.
This can be useful when combining for example a latex and a language
tool language server.

The last commit introduces support for highlighting empty selections
by properly calculating their vis ranges.
However File:content does return an empty string for empty ranges
and we can not check whether the diagnostic still applies to the same
position.

This is fixed by faking a one character range when receiving an
empty one.
Doing so we get a valid one-character content we can compare and
still have proper highlighting because we unconditionally decrement
the range end in highlight_diagnostics.

Fixes: 7099e56062861fc8dbb143e78447cce6bf886a97

Some language servers report diagnostics with empty ranges, start and
end are at the same line and character, which currently results in
no highlighting at all.

Since the end of a LSP range is exclusive we always subtract 1 from
it which results in invalid ranges when the server has sent an empty one.
Introduce a check to fix this special case.

`textDocument/publishDiagnostics` doesn't always have a `code` field,
resulting in `diagnostic.code` being nil and so it is replaced with
`diagnostic` in those cases.