Make the PATH configurable

[fpoli]

Currently, rustup needs to be in the PATH used by VS Code. This is not flexible enough for some use cases. See the comment here.

For improved flexibility, we could add an optional configuration field specifying the PATH to be used in place of the one available to VS Code. When such field is empty, Prusti Assitant should still use VS Code's PATH as it currently does.
It is necessary to configure the full PATH instead of just rustup's path, because Prusti internally searches for rustup in the PATH that we pass via Prusti Assistant.

Relevant source files:

• Declaration of the configuration fields: package.json
• Functions reading the configuration: config.ts
• Some of the files using rustup: rustup.ts, check.ts
• Some of the files using process.env: diagnostics.ts, server.ts

[jaivardhan-bhola]

Can I work on this?

[fpoli]

Sure! Let me know if you have any questions or suggestions.