[RFC Implementation] Merge Router into Core (#254)

* chore: merge router into core

* feat: use env as default mode

* chore: remove unused files

* feat: update router internals

* chore: update middleware internals

* chore: fix styling

* feat: update middleware to run on all http methods

* feat: add csrf method

* chore: fix styling

chore: fix styling

feat: revert nested config to flat config

chore: update readme

fix: patch up issue with custom error handlers

fix: add app instance to attach

chore: fix styling

Merge pull request #171 from Awilum/patch-9

update year

Merge pull request #170 from mychidarko/v3.x-dev

V3.x dev

Merge pull request #163 from leafsphp/v3.x-dev

V3.x dev