Add a versioned alias dart@2.16 for the current version (#107)

* Add a versioned alias dart@2.16 for the current version

A new check in Homebrew requires that the current version have
an alias with a version number, if we have other versions with numbers.
diff --git a/Aliases/dart@2.16 b/Aliases/dart@2.16
new file mode 120000
index 0000000..9f6c16d
--- /dev/null
+++ b/Aliases/dart@2.16
@@ -0,0 +1 @@
+../Formula/dart.rb
\ No newline at end of file