refactor: major overhaul of API

This refactor overhauls the API in preparation for v1.0.0.

Following major changes have been applied:
- Users now implement the Executor interface, instead of the Command
  interface. The Command object is now the container for a single
  command, created via the "New" factory.
- Configuration schema is read from the given Executor instance's type.
  Any field in that struct can be a potential flag or args target. The
  API now allows customization of such flags via struct field tags (to
  be documented). Configuration can be nested in multi-level structs for
  grouping and reuse.
- Commands can also be created via "MustNew" which panics if command
  creation fails.
- Better command hierarchy with clear definition of which flags are
  inherited from parent commands to child commands.
- Improved help & usage screens
- Better code & file structure
- Add golang CI linting

.github/workflows/validation.yaml

@@ -14,6 +14,10 @@ jobs:
       fail-fast: false
       matrix:
         go-version: [ '1.21', '1.22' ]
+    permissions:
+      contents: read
+      pull-requests: read
+      checks: write
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -26,5 +30,10 @@ jobs:
       - name: Download dependencies
         run: go mod download -x
 
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v6
+        with:
+          version: latest
+
       - name: Test
         run: go test ./...