avoid logical nullish assignment to allow node < v15 support

[doronpr]

fixes this error:

[04:42:31.066] Oops! Something went wrong! :(
[04:42:31.066] 
[04:42:31.066] ESLint: 8.33.0
[04:42:31.066] 
[04:42:31.066] /home/builduser/work/a5af963239e640d2/packages/package/node_modules/eslint-plugin-n/lib/util/get-typescript-extension-map.js:43
[04:42:31.066]         backward[javascript] ??= []
[04:42:31.066]                              ^^^
[04:42:31.066] 
[04:42:31.066] SyntaxError: Unexpected token '??='

[aladdin-add]

node.js < v16 has been dropped, so we won't accept this change. you can:

1. upgrade to node >=16. (has been EOL per https://github.com/nodejs/release#end-of-life-releases)
2. stay using eslint-pluign-n v15 if you are not able to upgrade as of now.

[scagood]

This would also break the mapping, it should be backward[javascript], like this:

Suggested change

	if (backward['javascript'] === null || backward['javascript'] === undefined) {
	backward['javascript'] = [];
	}
	if (backward[javascript] === null || backward[javascript] === undefined) {
	backward[javascript] = [];
	}

Also, I don't remember if we support == null, but this would work too

Suggested change

	if (backward['javascript'] === null || backward['javascript'] === undefined) {
	backward['javascript'] = [];
	}
	if (backward[javascript] == null) {
	backward[javascript] = [];
	}