accept JSON includes (#2265)

#2201 disabled using JSON as part of a bundle definition. I believe this
was not intended.

## Changes
Accept json files as includes, just as YAML files.
## Tests
Covered by the tests in #2201

acceptance/bundle/includes/non_yaml_in_include/output.txt

@@ -1,7 +1,7 @@
-Error: Files in the 'include' configuration section must be YAML files.
+Error: Files in the 'include' configuration section must be YAML or JSON files.
   in databricks.yml:5:4
 
-The file test.py in the 'include' configuration section is not a YAML file, and only YAML files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.
+The file test.py in the 'include' configuration section is not a YAML or JSON file, and only such files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.
 
 Name: non_yaml_in_includes
 

bundle/config/loader/process_root_includes.go

@@ -71,11 +71,11 @@ func (m *processRootIncludes) Apply(ctx context.Context, b *bundle.Bundle) diag.
 				continue
 			}
 			seen[rel] = true
-			if filepath.Ext(rel) != ".yaml" && filepath.Ext(rel) != ".yml" {
+			if filepath.Ext(rel) != ".yaml" && filepath.Ext(rel) != ".yml" && filepath.Ext(rel) != ".json" {
 				diags = diags.Append(diag.Diagnostic{
 					Severity:  diag.Error,
-					Summary:   "Files in the 'include' configuration section must be YAML files.",
-					Detail:    fmt.Sprintf("The file %s in the 'include' configuration section is not a YAML file, and only YAML files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.", rel),
+					Summary:   "Files in the 'include' configuration section must be YAML or JSON files.",
+					Detail:    fmt.Sprintf("The file %s in the 'include' configuration section is not a YAML or JSON file, and only such files are supported. To include files to sync, specify them in the 'sync.include' configuration section instead.", rel),
 					Locations: b.Config.GetLocations(fmt.Sprintf("include[%d]", i)),
 				})
 				continue