eval: add to osx platform (#2060)

pages/osx/eval.md

@@ -0,0 +1,11 @@
+# eval
+
+> Execute arguments as a single command in the current shell and return its result.
+
+- Call `echo` with the "foo" argument:
+
+`eval "{{echo foo}}"`
+
+- Set a variable in the current shell:
+
+`eval "{{foo=bar}}"`