reverting PR#445

[bourgeoa]

@ludwigschubi It seems that export interface ... extends ... do not export any js code because it is only declarative in typescript

PR reverting your PR#445 which had the following issues :

• issue #500
• and in building mashlib :

  
  WARNING in ../rdflib/esm/index.js 61:0-63:72
  "export 'AutoInitOptions' was not found in './fetcher'
  @ ./src/index.ts
  @ multi ./src/index.ts

WARNING in ../rdflib/esm/index.js 61:0-63:72 "export 'ExtendedResponse' was not found in './fetcher' @ ./src/index.ts @ multi ./src/index.ts

WARNING in ../rdflib/esm/index.js 61:0-63:72 "export 'FetchError' was not found in './fetcher' @ ./src/index.ts @ multi ./src/index.ts

[ludwigschub]

@bourgeoa i think #503 might fix it