Add tox file validation highlighting

[starlord-daniel]

Problem

Currently, issues with the tox file only occur when running the tox commands.
It would have a large benefit to get diagnostics that tell you where an issue is upon saving the file.

Solution

To implement diagnostics for tox, 2 things are needed:

• a tox validation mechanism
• VSCode diagnostics integration

There are some libraries, that do tox validation already, but a custom implementation for this would also work, leveraging the tox configuration specification.

To include diagnostics in VSCode, Programmatic Language Features can be used.

[The-Compiler]

Hm. I can see how that would be useful, but at the same time I'm not exactly keen to duplicate the whole tox.ini format specification in the plugin and keep it up to date.

There are some libraries, that do tox validation already

Do you perhaps have a link to those? I also wonder if we perhaps could parse the tox --help-ini output, but that seems rather brittle as well of course.