fix: load non relative tsconfig

[FRSgit]

fixes #267 (at least in some cases)

Currently, the loadTSCompilerOptions method does not handle correctly tsconfig.json "extends" field when it contains reference to an npm package.

Examples of such "extends": "extends": "@something/my-config/tsconfig.json" "extends": "@something/my-config/base.json" "extends": "@something/my-config"

With this change the script handles those cases correctly and (at least for me) fixes issue #267