feat: configure and inject environment variables in workers

[Angelmmiguel]

Configure environment variables via configuration files (TOML files with the same name of the worker). These variables will be configured in the vars section. Teh value can be static (string) or inherited from the current context ($VAR). In the second case, wws will initialized it to an empty string if the given environment variable doesn't exist in the current context.

This is an example of configuration file:

name = "test"
version = "1"

[vars]
MESSAGE = "Hello 👋! This message comes from an environment variable"

The usage of environment variables depends on the selected language:

• Rust: retrieve them using std::env
• JavaScript: environment variables are injected as global constants

You have all the information and examples in the documentation I added to this PR.

It closes #34

[assambar]

Greate feature! Just one small note on the docs.

[ereslibre]

Really great contribution. LGTM, a minor comment to get rid of mutability on the WASI context builder.