refactor(commands): separate commands and its core into two folders (#32)

- I think it would be better if one for commands only, one for the spec
handling core logic so people won't be confused by the core logic inside
the commands folder (cleaner I guess)
- Regen docs

commands/js/esbuild.go

@@ -1,14 +1,14 @@
 package js
 
 import (
-	"github.com/versenilvis/iris/commands/core"
+	"github.com/versenilvis/iris/spec"
 )
 
 func init() {
-	core.Register(&core.Spec{
+	spec.Register(&spec.Spec{
 		Name:        "esbuild",
 		Description: "An extremely fast JavaScript bundler",
-		Options: []core.Option{
+		Options: []spec.Option{
 			{Name: "--bundle", Description: "Bundle all dependencies into the output files"},
 			{Name: "--define", Description: "Replace variable names with a literal value, eg. --define:DEBUG=true"},
 			{Name: "--external", Description: "Exclude modules from the build"},