feat(cli-service): support plugin to inject process-env before load config file

[githoniel]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

We now support customizing the path to the configuration file via process.env.VUE_CLI_SERVICE_CONFIG_PATH, but this variable will be executed before the plugin apply. It means we can not set configuration file path in plugin.

I have developed a plugin cli-plugin-doc that registers the dev:doc command for starting project's documentation development. This cli-plugin-doc plugin requires the vue-cli build process, but needs to read ./docs/vue.config.js to implement the independent webpack build parameters compared with vue-cli-service serve.

With the current mechanism, neither defaultModes nor .env can meet the need.

Before:

module.exports.defaultModes = {
  'test:e2e': 'production'
}

process.env.NODE_ENV will be production when running vue-cli-service test:e2e

after:

module.exports.defaultModes = {
  'test:e2e':  ['production', {
     'VUE_CLI_SERVICE_CONFIG_PATH': './docs/vue.config.js'
   }]
}

In addition to the setting process.env.NODE_ENV when running vue-cli-service test:e2e , environment variables will be set process.env.VUE_CLI_SERVICE_CONFIG_PATH = './docs/vue.config.js'

environment variables is optional, so this does not break current plugins