[fix] default handlers for head

[dominikg]

see #3894

add default handling for HEAD requests. follow "get" handling but do not return body in the end.
custom head handlers in endpoint files are respected and take precedence over get.

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[Conduitry]

This code was already present, but I feel like .replace() isn't the best way to deal with this, for a few reasons. What if we just do something like:

Suggested change

	const method = event.request.method.toLowerCase().replace('delete', 'del'); // 'delete' is a reserved word
	let method = event.request.method.toLowerCase();
	if (method === 'delete') { // 'delete' is a reserved word
	method = 'del';
	}

[dominikg]

moved it to a util as it was used in two places

[Rich-Harris]

I just pushed a cosmetic change — the camelCased variables conflicted with the code style, and it was a bit of a wall-of-text, but it was tricky to do in a commit suggestion