Add and export utility classes for all types of nodes

[emilos]

Before:

    const statement = {
      type: 'IfStatement',
      test: {
        type: 'BinaryExpression',
        left: {
          type: 'MemberExpression',
          object: {
            type: 'Identifier',
            name: OBJECT_VARIABLE
          },
          property: { type: 'Identifier', name: 'language' }
        },
        right: { type: 'Literal', value: language },
        operator: '==='
      },
      consequent: {
        type: 'BlockStatement',
        body: [
          getTemplateAssignmentExpression(options.variables.template, { type: 'Literal', value: text.content })
        ]
      },
      alternate: null
    }

After:

    const statement = new IfStatement({
      test: new BinaryExpression({
        left: new MemberExpression({
          object: new Identifier({ name: OBJECT_VARIABLE }),
          property: new Identifier({ name: 'language' })
        }),
        right: new Literal({ value: language }),
        operator: '==='
      }),
      consequent: new BlockStatement({
        body: [
          getTemplateAssignmentExpression(options.variables.template, { type: 'Literal', value: text.content })
        ]
      })
    })

Those classes could throw if you forget to set a required property, e.g. BinaryExpression would throw if operator wouldn't be passed.