feat: Enhance #use directive for directories and cwd

The `#use` directive now supports referencing entire directories,
automatically including all `.myc` files within them. Additionally,
environments are now initialized with the current working directory as a
default search path, simplifying module resolution.

examples/sma.myc

@@ -1,10 +1,9 @@
-#use lib->sma
-;; Output: void
+#use rtl
 
 (do
   (def sma20 (SMA 20))
 
-  (def n 1000)
+  (def n 100)
   (while (> n 0)
     (do
       (print "val=" n " sma20=" (sma20 n))