add JS-snippet support using from: .module("/path/to/js-module.js") syntax

[sliemeobn]

allow to ship JS code as the source for imports (in addition to ".global" and explicitly provided imports)

@JSFunction(from: .module("/Modules/JSImportModule.mjs"))
func moduleAdd(_ lhs: Int, _ rhs: Int) throws(JSException) -> Int

@JSFunction(jsName: "renamedFunction", from: .module("/Modules/JSImportModule.mjs"))
func moduleRenamed() throws(JSException) -> String

@JSGetter(jsName: "version", from: .module("/Modules/JSImportModule.mjs"))
var moduleVersion: String

// Modules/JSImportModule.mjs
export function moduleAdd(lhs, rhs) {
    return lhs + rhs;
}

export function renamedFunction() {
    return "renamed";
}

export const version = "1.0";

I noticed that currently there is no diagnostic when from: is used in places that do not support it, I did not add these checks (eg: class members, setters) to this PR more focussed. (#793)