feat: Autocomplete Cargo-defined env vars in `env!` and `option_env!` (#12448)
Closes#12448
Important to know:
- Variables are taken from https://doc.rust-lang.org/cargo/reference/environment-variables.html and hardcoded as a const array.
- For the sake of simplicity I didn't include the autocompletion of `CARGO_BIN_EXE_<name>` and `OUT_DIR` since it would require information about build.rs and binary name. If somebody knows an easy way of obtaining them I can add those vars as well :)
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