Read configuration from the JSON file

[dvcopae]

[H] Xavier must check the mutation level for validity and give appropriate error messages in case the structure is invalid.

Initialize the mutation level class with the values read from the configuration JSON file.

[brokhiv]

The JSON attribute has the following type:

{type: "allow" | "block", mutation_operators: String[]} | String

Example ('Custom 1' from Jan Smits' paper):

"mutation_level": {
  "type": "block",
  "mutation_operators": [
    "BlockStatement",
    "ConditionalExpression===Tofalse",
    "ConditionalExpression===Totrue",
    "EqualityOperator===To!==",
    "ObjectLiteral",
    "Regex",
    "StringLiteralEmpty",
    "StringLiteralFill",
    "UnaryOperator+To-",
    "UnaryOperatorRemove~",
    "UnaryOperator-To+"
  ]
}

Example 2 (assuming "Custom 1" is a preset with the previous definition):

"mutation_level": "Custom 1"