Yarn configuration file variables aren't supported completely

[richard-underwood]

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npm-check-updates supports loading authentication tokens from .yarnrc.yml files. It also supports simple variables such as ${VAR} but does not support variables with fallback ${VAR-fallback} and ${VAR:-fallback}.

This is needed as a different authentication token is needed for publishing and supplied on the command line.

Steps to Reproduce

Set up a .yarnrc.yml using a variable for the authentication token for a scope:

npmScopes:
  xxxxxx:
    npmAlwaysAuth: true
    npmAuthToken: ${YARN_NPM_AUTH_TOKEN:-ghp_xxxxxxxxxxxxxxxxxxxxxxxxxxxx}
    npmRegistryServer: 'https://npm.pkg.github.com'

Current Behaviour

The variable isn't interpreted correctly, and authentication fails:

Using yarn
Checking /zzz/package.json
[====================] 47/47 100%
 @xxxxxx/package-name  unauthenticated: User cannot be authenticated with the token provided. 

Expected Behaviour

The variable is parsed correctly and authentication succeeds using the fallback value.