Add support for globalThis

[jpoehnelt]

Although global will likely always work in Apps Script, some developers may prefer to use globalThis because it abstracts environment away.

The globalThis property provides a standard way of accessing the global this value (and hence the global object itself) across environments. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis

[fossamagna]

@jpoehnelt Thank you for your contribution. Now, Identifier of default global object is global.
I agree with using globalThis better than global as default.

you can specify Identifier name of global object as follows:

const fs = require('fs');
const { generate } = require('gas-entry-generator');

const fooSource = fs.readFileSync('foo.js', {encoding: 'utf8'});
const options = {
  globalIdentifierName: "globalThis"
};
const output = generate(fooSource, options);
console.log(output.entryPointFunctions);