Add a Grunt task for detecting gutenberg_* names

[desrosj]

This adds a Grunt task that scans PHP files in the src directory for function gutenberg_ to help prevent scenarios where one is accidentally merged from the Gutenberg plugin.

Trac ticket: Core-56794

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Props desrosj, westonruter, ellatrix.

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[ellatrix]

Sounds good to me :)

[westonruter]

Suggested change

	function gutenberg_another_test(){
	function gutenberg_another_test() {

[westonruter]

I'm assuming this will be removed?

Suggested change

	function gutenberg_another_test(){
	}

[desrosj]

Yes, this was just added to demonstrate the task works. Will not be included in the final commit.

[westonruter]

Suggested change

	function gutenberg_test() {
	echo 'this is a test';
	}

[desrosj]

Same here. This was just added to demonstrate the task works. Will not be included in the final commit.