Add package docs and project vision/goals (#5169)

* Add goals/design vision to contributing doc
* Add barebones package documentation

pkg/javascript/vm.go

@@ -1,8 +1,8 @@
+// Package javascript provides the javascript runtime for the application.
 package javascript
 
 import (
 	"fmt"
-	"net/http"
 	"os"
 	"reflect"
 
@@ -10,12 +10,9 @@ import (
 	"github.com/stashapp/stash/pkg/logger"
 )
 
+// VM is a wrapper around goja.Runtime.
 type VM struct {
 	*goja.Runtime
-
-	Progress      chan float64
-	SessionCookie *http.Cookie
-	GQLHandler    http.Handler
 }
 
 // optionalFieldNameMapper wraps a goja.FieldNameMapper and returns the field name if the wrapped mapper returns an empty string.