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.

Fix lscp.open_file not actually adding to history

See merge request muhq/vis-lspc!45

Using the new latex lexer from the upcoming vis release I noticed
that vis-lspc default style id 43 overrides the environment style
from the lexer.

To prevent id clashing use the highest allowed id for lexer styles,
which is 64 according to the enum defined in vis/ui.h.

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