Add helper function for headers

[tobiasdiez]

Since its quite common to read the header strings, it would be nice if h3 could provide a helper function for this, which parses the headers and provides a structured way to access various common headers. For example, inspired by https://hapi.dev/module/accept/api/?v=5.0.2#parseallheaders one could have

parseHeaders(event).accept.mediaTypes
//  ["text/plain", "text/html", "application/json", "*/*"]

[NozomuIkuta]

@tobiasdiez @pi0

If you have not yet started working on this feature, I can submit a PR. How?

---

By the way, I'm imagining composables like below so that H3 methods can be aligned:

// Request phase
const headers = useHeaders(event) // all keys are lower-cased as `req.getHeaders()`
const value = useHeader(event, name) // `name` is case-insensitive as `req.getHeader(name)`
headers[name.toLowerCase()] === value // true
const body = useBody(event)

// Response phase
appendHeader(event, name, value)
send(event, data, type)
sendStream(event, data)

We might want to make useHeaders and useHeader available in response phase (i.e. on req) as well, by either making them kinda generic or providing dedicated composables like useReqHeader(s) and useResHeader(s) for easy typing.

[NozomuIkuta]

It seems that useBody has been deprecated in the code base.

h3/src/utils/body.ts

Lines 75 to 76 in 56bfe0a

	/** @deprecated Use `h3.readBody` */
	export const useBody = readBody

So, readHeader(s) or parseHeader(s) would be more suitable.

[pi0]

Sure thing @NozomuIkuta. I think we can introduce getHeaders(event), getHeader(event, name) and setHeader(event, name, value) and maybe appendHeader(event, name, value) utils.

[tobiasdiez]

Thanks @NozomuIkuta! These helper methods are good to have.

Strictly speaking, my proposal/idea went a bit further and included a structured access to the headers, i.e. parse them:

// Instead of
getHeader(event, "Content-Type")
// returns string: text/html; charset=UTF-8

// One has
getHeader(event, "Content-Type")
// returns object: {mediaTypes: ["text/html"], charsets: ["UTF-8"]}

Don't you think it makes sense and be helpful for devs to have this structured access?