Refactoring for custom module loader support

[tomatosalat0]

Motivation

To support fully custom IModuleLoader implementations, some code from the abstract ModuleLoader class has been moved into the public static ModuleFactory class.

With these public methods, custom IModuleLoader implementations can be created and used without deriving from the abstract class ModuleLoader. This allows custom code to implement certain features like a cache for parsed Esprima.Ast.Module instances which can be shared across multiple engine instances. The additional methods allow to use a different source specifier for modules instead of the default Uri.LocalPath variant used by the default implementation.

The behavior of the existing DefaultModuleLoader has not been changed.

Implementation

The static class allows to create Jint.Runtime.Modules.Module instances without publishing the actual implementations like SourceTextModule or SyntheticModule, so these implementations can still be changed without affecting the public API.

[lahma]

Splendid work, thank you for improving the public API promise and tweaking the logic!

[sebastienros]

Looks like the project got itself a new MVP. Thanks @tomatosalat0 for you recent PRs!