Extract classes to detect the type of specification into the core package

[derevnjuk]

To reduce the number of duplicates and provide a dedicated way to detect the type of specification we should extract several classes into the new core package.

e.g.

enum ImporterType {
  HAR = 'har',
  OASV2 = 'oasv2',
  OASV3 = 'oasv3',
  POSTMAN = 'postman'
}

interface Spec<T> {
  readonly type: ImporterType;
  readonly doc: T;
  readonly name?: string;
}

interface ImportOptions {
  readonly mimeType: string;
}

interface Importer {
  type: ImporterType;

  importSpec(
    content: string,
    options?: ImportOptions
  ): Promise<ImportedSpec | undefined>;
}