Add module jsfetch (#12531)

* Add module jsfetch for fetch support for JavaScript target https://developer.mozilla.org/docs/Web/API/Fetch_API

* Update lib/std/jsheaders.nim

* Update lib/std/jsformdata.nim

* Update lib/std/jsfetch.nim

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>
Co-authored-by: flywind <xzsflywind@gmail.com>

lib/std/jsheaders.nim

@@ -0,0 +1,83 @@
+## - HTTP Headers for the JavaScript target: https://developer.mozilla.org/en-US/docs/Web/API/Headers
+when not defined(js):
+  {.fatal: "Module jsheaders is designed to be used with the JavaScript backend.".}
+
+type Headers* = ref object of JsRoot ## HTTP Headers API.
+
+func newHeaders*(): Headers {.importjs: "new Headers()".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers
+
+func add*(self: Headers; key: cstring; value: cstring) {.importjs: "#.append(#, #)".}
+  ## Allows duplicated keys.
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/append
+
+func delete*(self: Headers; key: cstring) {.importjs: "#.$1(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/delete
+  ##
+  ## .. Warning:: Delete *all* items with `key` from the headers, including duplicated keys.
+
+func hasKey*(self: Headers; key: cstring): bool {.importjs: "#.has(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/has
+
+func keys*(self: Headers): seq[cstring] {.importjs: "Array.from(#.$1())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/keys
+
+func values*(self: Headers): seq[cstring] {.importjs: "Array.from(#.$1())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/values
+
+func entries*(self: Headers): seq[tuple[key, value: cstring]] {.importjs: "Array.from(#.$1())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/entries
+
+func `[]`*(self: Headers; key: cstring): cstring {.importjs: "#.get(#)".}
+  ## Get *all* items with `key` from the headers, including duplicated values.
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/get
+
+func `[]=`*(self: Headers; key: cstring; value: cstring) {.importjs: "#.set(#, #)".}
+  ## Do *not* allow duplicated keys, overwrites duplicated keys.
+  ## https://developer.mozilla.org/en-US/docs/Web/API/Headers/set
+
+func clear*(self: Headers) {.importjs:
+  "(() => { const header = #; Array.from(header.keys()).forEach((key) => header.delete(key)) })()".}
+  ## Convenience func to delete all items from `Headers`.
+
+func toCstring*(self: Headers): cstring {.importjs: "JSON.stringify(Array.from(#.entries()))".}
+  ## Returns a `cstring` representation of `Headers`.
+
+func `$`*(self: Headers): string = $toCstring(self)
+
+func len*(self: Headers): int {.importjs: "Array.from(#.entries()).length".}
+
+
+runnableExamples("-r:off"):
+
+  block:
+    let header: Headers = newHeaders()
+    header.add("key", "value")
+    assert header.hasKey("key")
+    assert header.keys() == @["key".cstring]
+    assert header.values() == @["value".cstring]
+    assert header["key"] == "value".cstring
+    header["other"] = "another".cstring
+    assert header["other"] == "another".cstring
+    assert header.entries() == @[("key".cstring, "value".cstring), ("other".cstring, "another".cstring)]
+    assert header.toCstring() == """[["key","value"],["other","another"]]""".cstring
+    header.delete("other")
+    assert header.entries() == @[("key".cstring, "value".cstring)]
+    header.clear()
+    assert header.entries() == @[]
+    assert header.len == 0
+
+  block:
+    let header: Headers = newHeaders()
+    header.add("key", "a")
+    header.add("key", "b")  ## Duplicated.
+    header.add("key", "c")  ## Duplicated.
+    assert header["key"] == "a, b, c".cstring
+    header["key"] = "value".cstring
+    assert header["key"] == "value".cstring
+
+  block:
+    let header: Headers = newHeaders()
+    header["key"] = "a"
+    header["key"] = "b"  ## Overwrites.
+    assert header["key"] == "b".cstring