Support of the .sld file extension for Scheme.

lib/linguist/languages.yml

@@ -1552,6 +1552,7 @@ Scheme:
   color: "#1e4aec"
   primary_extension: .scm
   extensions:
+  - .sld
   - .sls
   - .ss
   interpreters:

samples/Scheme/basic.sld

@@ -0,0 +1,7 @@
+(define-library (libs basic)
+    (export list2 x)
+    (begin
+        (define (list2 . objs)  objs)
+        (define x 'libs-basic)
+        (define not-exported 'should-not-be-exported)
+        ))