Test processing of set-cookie headers in SvelteKit response

[H0R5E]

This PR adds a vitest suite and a test harness to allow testing of serverless.js. The files serverless.js and shim.js are copied to a folder where index.js and manifest.js are stored as stubs in the parent folder. They can then be used within the Vitest testing framework.

The tests here check that set-cookie headers are converted to multiValueHeaders in the lambda response, while any other header remains in the header field, being comma separated if multiple values are set.

This is an addon to #32, if desirable.