Support for assigning variables to other variables, printing variables, and commenting

inputs/printing_program.bas

@@ -0,0 +1,8 @@
+10 REM Assign vars
+20 LET a$="Hello"
+30 LET b$="World"
+40 LET c$=a$
+50 REM Print values
+60 PRINT a$
+70 PRINT b$
+80 PRINT c$

inputs/simple_program.bas

@@ -1,3 +1,4 @@
 10 PRINT "Hello World"
 20 LET apple=10
-30 LET b$="Hello"
+30 LET b$="Hello"
+40 LET cat=apple