Compatibility for Jest 28

[EvHaus]

jest-handlebars module fails in Jest 28 with this error:

Invalid return value:
`process()` or/and `processAsync()` method of code transformer found at 
"/Users/myapp/node_modules/jest-handlebars/index.js" 
should return an object or a Promise resolving to an object. The object 
must have `code` property with a string of processed code.
This error may be caused by a breaking change in Jest 28:
https://jestjs.io/docs/upgrading-to-jest28#transformer
Code Transformation Documentation:
https://jestjs.io/docs/code-transformation

This is due to the change here: https://jestjs.io/docs/upgrading-to-jest28#transformer

Unfortunately this PR will make jest-handlebars no longer compatible with Jest <=27, so it would need to be published a new major version change.

You can test this PR by installing it your project via this npm dependency definition:

"devDependencies": {
    "jest-handlebars": "fetzi/jest-handlebars#pull/3/head"
}

[jaylinski]

If you also want coverage, use this:

const Handlebars = require('handlebars');

module.exports = {
    process(sourceText, sourcePath) {
        const { code, map } = Handlebars.precompile(sourceText, { srcName: sourcePath });

        return {
            code: `
                const Handlebars = require('handlebars');
                module.exports = Handlebars.template(${code});
            `,
            map,
        };
    },
};