all: add $res compile time function to get returned value in defer block (#18382)

doc/docs.md

@@ -2119,6 +2119,42 @@ fn main() {
 }
 ```
 
+To access the result of the function inside a `defer` block the `$res()` expression can be used.
+`$res()` is only used when a single value is returned, while on multi-return the `$res(idx)`
+is parameterized.
+
+```v ignore
+fn (mut app App) auth_middleware() bool {
+	defer {
+		if !$res() {
+			app.response.status_code = 401
+			app.response.body = 'Unauthorized'
+		}
+	}
+	header := app.get_header('Authorization')
+	if header == '' {
+		return false
+	}
+	return true
+}
+
+fn (mut app App) auth_with_user_middleware() (bool, string) {
+	defer {
+		if !$res(0) {
+			app.response.status_code = 401
+			app.response.body = 'Unauthorized'
+		} else {
+			app.user = $res(1)
+		}
+	}
+	header := app.get_header('Authorization')
+	if header == '' {
+		return false, ''
+	}
+	return true, 'TestUser'
+}
+```
+
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