…ass extending a d.ts class (#2537)

* add test

* improve test

* add comment

* new variant method for DTS

* fix issue

* typo and small adjustment

* refactor to deal with some exceptions

* add symbols dispose and asyncDispose

* support some text region support in d.ts files

* refactor to avoid inconsistencies

* special check regarding computed property names

* also check fields, getters, setters

* small refactoring