chore: change OptionType to alias for usage as string (#377)

To be backwards compatible using this repo as a library, use a string
alias. This allows using the 'type' as a `string` or the new enum.
Since enum validation is quite weak in Go, this is baiscally
functionally equvialent.

Author: Emyrk

provider/formtype.go

@@ -13,7 +13,7 @@ import (
 //
 // The value have to be string literals, as type constraint keywords are not
 // supported in providers.
-type OptionType string
+type OptionType = string
 
 const (
 	OptionTypeString     OptionType = "string"