Custom mask for secret (#970)

* Masking Character added, not yet used.

* Setting the masking character can be chained with other extensions.

* Added string extension for masking, and replaced hardcoded asterisks.

* Check if mask is null first.

* Fixed Typo in previous test and added new test for custom masks.

* Added tests for masking with null character

* Added docs and example.

* Adjusted extensions so that Mask is integrated into Secret extension. Updated Exampls and Tests accordingly

docs/input/prompts/text.md

@@ -63,6 +63,23 @@ What's the secret number? _
 
 ```text
 Enter password: ************_
+```
+
+## Masks
+
+<?# Example symbol="M:Prompt.Program.AskPasswordWithCustomMask" project="Prompt" /?>
+
+
+```text
+Enter password: ------------_
+```
+
+You can utilize a null character to completely hide input.
+
+<?# Example symbol="M:Prompt.Program.AskPasswordWithNullMask" project="Prompt" /?>
+
+```text
+Enter password: _
 ```
 
 ## Optional