implemented 'IF' BASIC command

src/basic/basic_commands/cmd__unimplemented.s

@@ -21,8 +21,6 @@ cmd_input:
 	nop
 cmd_read:
 	nop
-cmd_if:
-	nop
 cmd_gosub:
 	nop
 cmd_return:

src/basic/basic_commands/cmd_if.s

@@ -0,0 +1,29 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+
+cmd_if:
+
+	// Calculate expression
+
+	jsr FRMEVL
+
+	// Check the result - either string size or floating point 0 vs non-0
+
+	lda FAC1_exponent
+	beq_16 cmd_rem
+
+	// Condition fulfilled - look for GOTO or THEN
+
+	jsr fetch_character_skip_spaces
+	cmp #$89                           // 'GOTO'
+	beq_16 cmd_goto
+
+	cmp #$A7                           // 'THEN'
+	bne_16 do_SYNTAX_error
+
+	pla
+	pla
+
+	jmp execute_statements