Replace isValid() calls with assert()

There's more value on showing how `assert()` displays the validation
messages than simply showing if `isValid()` returns `true` or `false`.

However, that increases the chances of having outdated documentation, so
I created a doc linter that updates the Markdown files with the
correct message.

docs/validators/Executable.md

@@ -5,7 +5,11 @@
 Validates if a file is an executable.
 
 ```php
-v::executable()->isValid('script.sh'); // true
+v::executable()->assert('/path/to/file');
+// → "/path/to/file" must be an executable file
+
+v::executable()->assert('/path/to/executable');
+// Validation passes successfully
 ```
 
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