feat: support env variable substitution

[altitudems]

Motivation

We can't store sensitive information like Netlify API keys in .vscode/settings.json for security reasons.

We would like to use environment variables instead.

Changes

Handles env variable substitution for some contributed config variables.

_Note: this only handles the ${env:MY_VAR} case, and does not try to handle every type of substitution._

e.g.

{
  "netlify.api_token": "${env:MY_NETLIFY_PAT}",
  "netlify.build_hook": "${env:MY_NETLIFY_BUILD_HOOK}",
  "netlify.site_id": "${env:MY_NETLIFY_SITE_ID}"
}

Alternatives

If upstream VSCode presents a way to handle variables substitution for contributed config, this might not be needed. See: https://github.com/Microsoft/vscode/issues/46471

We could take a more drastic approach and encrypt the settings.json using something like git secret.

Other attempts at patching this: https://github.com/idleberg/node-vscode-get-config https://github.com/DominicVonk/vscode-variables

[shailen-naidoo]

@altitudems Hey man! I really appreciate your work that you have put in here, I'll have a look at this tonight and get back to you :smile:

[shailen-naidoo]

@altitudems Can you please add a how-to test so that I have a direction for validating what you have worked on

[altitudems]

Sure thing.

1. Create a Netlify PAT
2. Store the PAT as a env variable

   
   # In your ~/.zshrc or ~/.bashrc file add:
   export MY_NETLIFY_PAT=[your-pat-here]

Repeat for other sensitive values

3. Restart VSCode so that it loads in the new env variable
4. Modify `settings.json`, adding a reference to the environment variables using `${env:YOUR_ENV-VAR}`
```json
{
  "netlify.api_token": "${env:MY_NETLIFY_PAT}",
  "netlify.build_hook": "${env:MY_NETLIFY_BUILD_HOOK}",
  "netlify.site_id": "${env:MY_NETLIFY_SITE_ID}"
}

5. Confirm that the Netlify extension works as expected (it will log the substituted values in the output panel)

[shailen-naidoo]

@altitudems I am happy with your changes, are you perhaps able to write a unit test for it?