Make ENDL optional like ENDSECTION (#1538)

Add warning for `LOAD` without `ENDL`

test/asm/load-endings.asm

@@ -0,0 +1,51 @@
+MACRO data
+	db SECTION(@), \#
+ENDM
+
+MACRO now_in
+	if strcmp("\1", "nothing")
+		assert !strcmp(SECTION(@), \1)
+	else
+		assert !def(@)
+	endc
+ENDM
+
+now_in nothing
+
+SECTION "A", ROM0
+	now_in "A"
+	data 1
+	LOAD "P", WRAM0
+		now_in "P"
+		data 2
+
+	; LOAD after LOAD
+	LOAD "Q", WRAM0
+		now_in "Q"
+		data 3
+
+; SECTION after LOAD
+SECTION "B", ROM0
+	now_in "B"
+	data 4
+	LOAD "R", WRAM0
+		now_in "R"
+		data 5
+
+		; PUSHS after LOAD
+		PUSHS
+			SECTION "C", ROM0
+				now_in "C"
+				data 6
+				LOAD "S", WRAM0
+					now_in "S"
+					data 7
+
+		; POPS after LOAD
+		POPS
+		now_in "R"
+		data 8
+
+; ENDSECTION after LOAD
+ENDSECTION
+now_in nothing