user defined variables for all .json configuration files

[Lakshmikanth2001]

This PR fixes #2809

	const configuration = {
		'params': {
			'git_version': '2.30.0',
			'node_version': '14.16.1',
			'python_version': '3.8',
			'port': '5858',
		},
		'node_version': '${params:node_version}',
		'port': '${params:port}',
		'virtual_env': 'virtualenv .venv --python=python${params:python_version}'
	};
		assert.deepStrictEqual({ ...result }, {
			'params': {
				'git_version': '2.30.0',
				'node_version': '14.16.1',
				'python_version': '3.8',
				'port': '5858',
			},
			'node_version': '14.16.1',
			'port': '5858',
			'virtual_env': 'virtualenv .venv --python=python3.8'
		});

This is the expected behaviour

[ghost]

All CLA requirements met.

[sandy081]

I am not the owner of Configuration Resolver. Probably @weinand / @isidorn (before) ?

[alexr00]

This actually is for me. I mistakenly read it as configuration, not configuration resolver.

[alexr00]

@Lakshmikanth2001 thank you for the pull request. Since ${} style variables are only set by VS Code and are uniform (i.e., the variables evaluate to the same thing no matter where they are used), it is confusing to have user-defined variables that can change per file. Additionally, it wouldn't work with the json schema for each file.

Instead, if you want the have some kind of variable expansion, you can set environment variables.