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/jsformdata.nim

@@ -0,0 +1,65 @@
+## - `FormData` for the JavaScript target: https://developer.mozilla.org/en-US/docs/Web/API/FormData
+when not defined(js):
+  {.fatal: "Module jsformdata is designed to be used with the JavaScript backend.".}
+
+type FormData* = ref object of JsRoot ## FormData API.
+
+func newFormData*(): FormData {.importjs: "new FormData()".}
+
+func add*(self: FormData; name: cstring; value: SomeNumber | bool | cstring) {.importjs: "#.append(#, #)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/append
+  ## Duplicate keys are allowed and order is preserved.
+
+func add*(self: FormData; name: cstring; value: SomeNumber | bool | cstring, filename: cstring) {.importjs: "#.append(#, #, #)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/append
+  ## Duplicate keys are allowed and order is preserved.
+
+func delete*(self: FormData; name: cstring) {.importjs: "#.$1(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/delete
+  ##
+  ## .. Warning:: Deletes *all items* with the same key name.
+
+func getAll*(self: FormData; name: cstring): seq[cstring] {.importjs: "#.$1(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/getAll
+
+func hasKey*(self: FormData; name: cstring): bool {.importjs: "#.has(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/has
+
+func keys*(self: FormData): seq[cstring] {.importjs: "Array.from(#.$1())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/keys
+
+func values*(self: FormData): seq[cstring] {.importjs: "Array.from(#.$1())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/values
+
+func pairs*(self: FormData): seq[tuple[key, val: cstring]] {.importjs: "Array.from(#.entries())".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/entries
+
+func put*(self: FormData; name, value, filename: cstring) {.importjs: "#.set(#, #, #)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/set
+
+func `[]=`*(self: FormData; name, value: cstring) {.importjs: "#.set(#, #)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/set
+
+func `[]`*(self: FormData; name: cstring): cstring {.importjs: "#.get(#)".}
+  ## https://developer.mozilla.org/en-US/docs/Web/API/FormData/get
+
+func clear*(self: FormData) {.importjs:
+  "(() => { const frmdt = #; Array.from(frmdt.keys()).forEach((key) => frmdt.delete(key)) })()".}
+  ## Convenience func to delete all items from `FormData`.
+
+func toCstring*(self: FormData): cstring {.importjs: "JSON.stringify(#)".}
+
+func `$`*(self: FormData): string = $toCstring(self)
+
+func len*(self: FormData): int {.importjs: "Array.from(#.entries()).length".}
+
+
+runnableExamples("-r:off"):
+  let data: FormData = newFormData()
+  data["key0"] = "value0".cstring
+  data.add("key1".cstring, "value1".cstring)
+  data.delete("key1")
+  assert data.hasKey("key0")
+  assert data["key0"] == "value0".cstring
+  data.clear()
+  assert data.len == 0