refactor!: use DefinePlugin (again) instead of EnvironmentPlugin

[haoqunjiang]

Fixes #4658

The reasoning behind the previous change was stated at #3579.

But after a closer look at the webpack documentation, I see that webpack is only against the process: { env: { NODE_ENV: JSON.stringify('production') } } style, which would break the process object mock, while, in Vue CLI's base config, only process.env is replaced, which has a much smaller impact.

By providing the whole process.env object, use cases such as the abovementioned destructuring of process.env are now supported, which is more intuitive IMO.

What might be problematic is how users customize their env vars that not started with VUE_APP. If they follow our example, they may accidentally override the process.env object - we've made that mistake too

vue-cli/packages/@vue/cli-service/lib/commands/build/resolveWcConfig.js

Lines 56 to 62 in 02859cf

	config
	.plugin('web-component-options')
	.use(require('webpack/lib/DefinePlugin'), [{
	'process.env': {
	CUSTOM_ELEMENT_NAME: JSON.stringify(libName)
	}
	}])

Letting users tap into our base config is not ideal, either. Because such configuration is very tedious and tends to break between major versions like we already have done.
For this issue, we can later add a section in the documentation guiding them on how to correctly define a custom environment variable - by adding another DefinePlugin instance and use the 'process.env.SOME_NAME': '"somevalue"' syntax.

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:

[haoqunjiang]

/cc @LinusBorg