warning diagnostics for unused locals

examples/packages/qualified_shadow/app/main.bro

@@ -1,6 +1,6 @@
 import "../math"
 
-read func(value i8) int {
+read func(_ i8) int {
 	return math.value
 }
 

examples/programs/conditional_unwrap/main.bro

@@ -33,7 +33,7 @@ main func() i32 {
 
 	# optional pointer none -> skipped
 	z ?@i32 = none
-	if z |q| {
+	if z |_| {
 		total = total + 1000
 	}
 

examples/programs/unused_locals/main.bro

@@ -0,0 +1,12 @@
+warn_only func(value i32, unused i32) i32 {
+	local i32 = 1
+	write_only i32 = 2
+	write_only = 3
+	consumed i32 = value
+	_ = consumed
+	return value
+}
+
+main func() i32 {
+	return warn_only(7, 9)
+}