Add more tests of lexer's skipToLeadingKeyword behavior (#1971)

The character after the ending keyword gets blue-painted.

test/asm/leading-keywords.asm

@@ -0,0 +1,20 @@
+; This covers some edge cases of `skipToLeadingKeyword`
+
+SECTION "test", ROM0
+
+MACRO mac
+	if 1
+		DEF x = \1_WIDTH
+		DEF y = \1_HEIGHT
+	elif 0
+		DEF x = \1_HEIGHT
+		DEF y = \1_WIDTH
+	else
+		fail "nope"
+	endc
+	db \1_WIDTH, \1_HEIGHT, x, y
+ENDM
+
+	DEF NAME_WIDTH EQU 10
+	DEF NAME_HEIGHT EQU 9
+	mac NAME