don't initialize body scroll when in fastboot

[jamemackson]

This PR addresses a " TypeError: window.addEventListener is not a function" error when using ember-mobile-menu under fastboot by preventing the body-scroll-lock library from initializing when running on the server where window isn't defined.

Error & start of stack trace...

There was an error running your app in fastboot. More info about the error: 
 TypeError: window.addEventListener is not a function
    at eval (webpack://__ember_auto_import__/./node_modules/body-scroll-lock/lib/bodyScrollLock.esm.js?:10:147)
    at Module../node_modules/body-scroll-lock/lib/bodyScrollLock.esm.js....

Fixes #186

[jamemackson]

so turns out this doesn't yet work. Here's a minimal representation of the issue. https://github.com/jamemackson/ember-mobile-menu-fastboot-example

@nickschot any chance you can take a peek and give me a nudge in the right direction?

[nickschot]

Suggested change

	fastboot = getOwner(this).lookup('service:fastboot');
	get isFastBoot() {
	return !!this.fastboot?.isFastBoot;
	}
	get fastboot() {
	return getOwner(this).lookup('service:fastboot');
	}
	get isFastBoot() {
	return !!this.fastboot?.isFastBoot;
	}

To be in line with the updated syntax on master.

[bakerac4]

@nickschot would it be possible to get this merged? Im also seeing this in fastboot.

[nickschot]

Closing, followup in #891 and #854