Make the extension extensible by third-parties

[fcollonval]

Is your feature request related to a problem? Please describe.

Currently the extension is not extensible on the following subject:

• The search engine - hard coded ripgrep usage on the backend
• The file opener and replacer - hard coded FileEditor to highlight and/or replace search matches from ripgrep.

Describe the solution you'd like

It would be good to allow third-parties to register plugin to alleviate those limitations.

An API candidate is the following:

interface ISearchReplaceProvider {
  /**
   * Initialize the provider with the search parameters
   * @returns Additional exclude filters to be set on ripgrep search or null to skip calling ripgrep.
   */
  preprocess: (all search parameters) => string[] | null;
  /**
   * Update/Extend the matches from ripgrep search
   * This could add file matches for excluded files in the preprocessor phase
   * or add a customized `displayAndReplace` callback on file matches
   */
  postprocess: (filematches: IFileMatch[]) => IFileMatch[];
}

interface ISearchReplaceRegistry {
  /**
   * Add a provider
   * Options
   *   rank: Rank of the provider (default order of registration)
   */
  add: (provider: ISearchReplaceProvider, options?: {rank: number}) => void;
}


  /**
   * Interface to represent matches in a file
   */
  export interface IFileMatch {
    /**
     * path of file
     */
    path: string;
    /**
     * all matches within that file
     */
    matches: IMatch[];
    /**
     * Open and optionally replace matches in a file
     * ### Notes
     *   If replace is false, only open the file and highlight to the first match
     *   The replace action should be undoable
     * @param path File path
     * @param matches Matches within the file
     * @param replace Whether to replace or not
     * @returns the widget opened
     */
    displayAndReplace?: (path: string, matches?: SearchReplace.IMatch[], replace = false) => Promise<Widget>;
  }