Add offline functionality

[wav3m1nd3d]

• VSCodium server on start requires internet connection to check whether installed version is up-to-date. This behaviour is unnecessary: the server can try to start vscodium and on a crash or version mismatch one can try to pull version from internet (in terms of extension behaviour),but a custom extensions would be better.

• Poetry doesn't store it's own virtual environment (everytime one needs poetry shell and poetry install + reload VSCodium window)

[wav3m1nd3d]

For now the possibility of creation of a custom extension (or as meant previously, a fork of openssh) makes no sense because:

1. It supports offline functionality by checking of previous SERVER_SCRIPT presence
2. It's a way of coping with issues like version mismatch itself or long rebuild times due to poor caching and wrong layer invalidation. The error in the approach is taking such issues as facts instead of something that can be solved.

It would be better to concentrate on solving version mismatch (Issue #18) and rebuild times enhancement (Issues #5, #7, #16) to make this issue negligible.

[wav3m1nd3d]

Considered closed due to listed issues completion and #7 requiring another check