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

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.

Since the diagnostics table is created when we open the file
it will always be ~= nil.

* fix typos
* wrap file or project names in code blocks
* drop the hint taht vis-lspc is in an early state
* s/json/JSON/