Implement EXPORT DEF to define and export symbols (#1422)

test/asm/export.asm

@@ -0,0 +1,18 @@
+EXPORT undefined
+
+DEF equ_sym EQU 1
+DEF var_sym = 2
+EXPORT equ_sym, var_sym
+
+EXPORT DEF constant EQU 42
+EXPORT DEF variable = 1337
+EXPORT DEF byte RB
+EXPORT DEF word RW
+EXPORT DEF long RL
+EXPORT REDEF constant EQU 69
+EXPORT REDEF variable = 1234
+
+; String constants can't be exported or imported.
+DEF equs_sym EQUS "hello"
+EXPORT equs_sym ; exports undefined symbol `hello` due to EQUS expansion
+EXPORT DEF string EQUS "goodbye" ; invalid syntax