fix: improve handling of the env! macro

[dpaoliello]

First issue:

Currently cargo_config_env assumes that cargo config always serializes each entry in the env table to an object, but it is valid to use just a string for the value in the env table and this is converted to a string in the JSON:

> type .\.cargo\config.toml
[env]
IS_OBJECT = { value = "Hello" }
NOT_OBJECT = "World!"
> cargo +nightly -Z unstable-options config get --format json
{"env":{"IS_OBJECT":{"value":"Hello"},"NOT_OBJECT":"World!"}}

The fix for this is to handle JSON strings as well as objects.

Second issue:

Currently the only values available to the env! macro are those set in the env table of the Cargo configuration file (plus a few other hard coded values). This makes it difficult to simulate having environment variables set (e.g., to match a CI system) without either modifying the current project's configuration (which is usually checked-in) or the user's configuration (which is shared between projects).

This change makes the values set in the cargo.extraEnv configuration option available via the env! macro when loading a project via Cargo manifests. This provides a straightforward way for users to set custom values for env! and for those values to be consistent between language services and cargo check.

Note that this feature respects the force option in the env table, so it will only overwrite the values from extraEnv if force is set to true.

Fixes #13004
Partial fix for #17593 (changes env! but not include!)

[ChayimFriedman2]

Just one nit.

Thanks!

[ChayimFriedman2]

Env implements Extend, please use that.

[ChayimFriedman2]

Here again use Extend.

[ChayimFriedman2]

Thanks!