Add 4DProject and 4DForm extensions to JSON (#4802)

* Update languages.yml

I would like to add, to complete the 4D programming language, specific extensions, linked to JSON & XML, for the other files used into 4D projects repositories.
Samples into this repository: https://github.com/vdelachaux/4DPop

- JSON
".4DProject": https://github.com/vdelachaux/4DPop/blob/master/Project/4DPop.4DProject
".4DForm": https://github.com/vdelachaux/4DPop/blob/master/Project/Sources/Forms/ASSISTANT/form.4DForm

- XML
.4DCatalog: https://github.com/vdelachaux/4DPop/blob/master/Project/Sources/catalog.4DCatalog
.4DSettings: https://github.com/vdelachaux/4DPop/blob/master/Project/Sources/settings.4DSettings

* Remove 4DCatalog and 4DSettings from XML

This isn't needed as the files are correctly detected as XML already

* Add samples

Source: https://github.com/vdelachaux/4DPop-Git
License: MIT

* Replace with smaller sample

Source: https://github.com/4D-JP/4d-test-read-only-fields/blob/b54a3ce77eefd1e825356b0fdad70b18baf6ed1c/read-only-fields.4dbase/Project/Sources/TableForms/1/Input/form.4DForm
License: MIT

Co-authored-by: Colin Seymour <colin@github.com>
Co-authored-by: Colin Seymour <colin@symr.io>

lib/linguist/languages.yml

@@ -2887,6 +2887,8 @@ JSON:
   - topojson
   extensions:
   - ".json"
+  - ".4DForm"
+  - ".4DProject"
   - ".avsc"
   - ".geojson"
   - ".gltf"

samples/JSON/4DPopGit.4DProject

@@ -0,0 +1,3 @@
+{
+	"$comment": "The project file let you override the location for most folders"
+}