JSX V4 (#547)

* react ppx v4 and parsing spread props

* change the empty record to "_" for jsx upper case

* change empty record to {key: None} for jsx upper case

* polishing the loc of react ppx

* implicit option type of @obj record update

* add match vbs for optional arg with default value

* add @optional record declarations

* fix missing pattern

* clean up codes and comments

* support React.forwardRef

* replace filterMap with std lib

* Add JSX V4 spec.

* typo

* JSX spec: move createElement to application.

Following https://forum.rescript-lang.org/t/jsx-v4-next-rescript-react-0-10-x/3431/10

* spec: comment on name

* spec: clean up file

* external

* Be consistent with interface

only show how to transform the arrow function.

* Update JSXV4.md

* restore React.createElement in application site

* spec: react fragment

* spec: tweak fragment.

* make react component name capitalized

* test: add test files

* fix ppx for forwardRef

* remove ref from props except forwardRef

* add test for forwardRef

* format

* remove uneccessary adding @optional attr from extracting props type

* indent by relocating every fn from jsxMapper

* remove unused jsxVersion

* jsx version check is conducted in compiler config side

* add new line

* update jsx ppx comment

* rename v3 -> v4, restore v3, add cli args

* add tests for v3 and v4 respectively

Co-authored-by: Cristiano Calcagno <cristianoc@users.noreply.github.com>

Author: mununki

cli/JSXV4.md

@@ -0,0 +1,100 @@
+**Abbreviation**
+
+The placement of `@react.component` is an abbreviation as described below.
+
+**Normal Case**
+
+```rescript
+@react.component
+let make = (~x, ~y, ~z) => body
+
+// is an abbreviation for
+
+let make = @react.component (~x, ~y, ~z) => body
+```
+
+**Forward Ref**
+
+```rescript
+@react.component
+let make = React.forwardRef((~x, ~y, ref) => body)
+
+// is an abbreviation for
+
+let make = React.forwardRef({
+  let fn =
+    @react.component (~x, ~y, ~ref=?) => {
+      let ref = ref->Js.Nullable.fromOption
+      body
+    }
+  (props, ref) => fn({...props, ref: {ref->Js.Nullable.toOption}})
+})
+```
+
+**Conversion**
+
+Conversion applies to an arrow function definition where all the arguments are labelled.
+It produces a type definition and a new function.
+
+**Definition**
+
+```rescript
+@react.component (~x, ~y=3+x, ?z) => body
+
+// is converted to
+
+type props<'x, 'y, 'z> = {x: 'x, @optional y: 'y, @optional z: 'z, @optional key: string}
+
+({x, y, z}: props<_>) => {
+  let y = switch props.y {
+  | None => 3 + x
+  | Some(y) => y
+  }
+  body
+}
+```
+
+**Application**
+
+```rescript
+<Comp x>
+// is converted to
+React.createElement(Comp.make, {x})
+
+<Comp x y=7 ?z>
+// is converted to
+React.createElement(Comp.make, {x, y:7, @optional z})
+
+<Comp x key="7">
+// is converted to
+React.createElement(Comp.make, {x, key: "7"})
+```
+
+**Interface And External**
+
+```rescript
+@react.component (~x: int, ~y: int=?, ~z: int=?) => React.element
+
+// is converted to
+
+type props<'x, 'y, 'z> = {x: 'x, @optional y: 'y, @optional z: 'z}
+
+props<int, int, int> => React.element
+```
+
+Since an external is a function declaration, it follows the same rule.
+
+**Component Name**
+
+Use the V3 convention for names, and make sure the generated
+function has the name of the enclosing module/file.
+
+**Fragment**
+
+```rescript
+<> comp1 comp2 comp3 </>
+
+// is converted to
+
+ReactDOMRe.createElement(ReasonReact.fragment, [comp1, comp2, comp3])
+```