checker: support a custom [deprecated: 'message']

vlib/v/checker/checker.v

@@ -1784,7 +1784,13 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			call_expr.pos)
 	}
 	if f.is_deprecated {
-		c.warn('function `$f.name` has been deprecated', call_expr.pos)
+		mut deprecation_message := 'function `$f.name` has been deprecated'
+		for d in f.attrs {
+			if d.name == 'deprecated' && d.arg != '' {
+				deprecation_message += '; $d.arg'
+			}
+		}
+		c.warn(deprecation_message, call_expr.pos)
 	}
 	if f.is_unsafe && !c.inside_unsafe
 		&& (f.language != .c || (f.name[2] in [`m`, `s`] && f.mod == 'builtin')) {

vlib/v/checker/tests/use_deprecated_function_warning.out

@@ -1,6 +1,13 @@
-vlib/v/checker/tests/use_deprecated_function_warning.vv:3:5: error: function `os.cp_r` has been deprecated
-    1 | import os
-    2 | fn main() {
-    3 |     os.cp_r('./aa', './bb', true)
-      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~
-    4 | }
+vlib/v/checker/tests/use_deprecated_function_warning.vv:12:2: error: function `xyz` has been deprecated
+   10 | 
+   11 | fn main() {
+   12 |     xyz()
+      |     ~~~~~
+   13 |     abc()
+   14 | }
+vlib/v/checker/tests/use_deprecated_function_warning.vv:13:2: error: function `abc` has been deprecated; use foo2 instead
+   11 | fn main() {
+   12 |     xyz()
+   13 |     abc()
+      |     ~~~~~
+   14 | }

vlib/v/checker/tests/use_deprecated_function_warning.vv

@@ -1,4 +1,14 @@
-import os
-fn main() {
-	os.cp_r('./aa', './bb', true)
+[deprecated]
+fn xyz() {
+	println('hi')
+}
+
+[deprecated: 'use foo2 instead']
+fn abc() {
+	println('hi')
+}
+
+fn main() {
+	xyz()
+	abc()
 }