feat: add comment support to saccharine language

Add '#' comment syntax that works like Python comments.
Comments can take up a whole line or appear at the end of a line.
All characters after '#' until the next newline or EOF are ignored.

Closes #24

tests/comments.expected

@@ -0,0 +1 @@
+VALUE

tests/comments.test

@@ -0,0 +1,15 @@
+# This is a full-line comment at the start
+# The following defines the identity function
+identity := \x.x # This is an end-of-line comment
+
+# Define a simple function that applies a function twice
+twice := \f.\x.(f (f x))
+
+# Test that comments don't interfere with expressions
+result := (twice identity VALUE) # Should just return VALUE
+
+# Multiple comments in a row
+# can appear anywhere
+# without breaking the code
+
+result # Final comment at the end