Add an `undef` global Jinja function (#75435)

* add tests for fail filter

also tests that fail does not block inspectability

* add fail filter

fallback message is a bit clunky,
since you can't invoke a filter without specifying an input.
That is, "{{ fail }}" doesn't work,
so you have to do "{{ None | fail }}"

* document 'fail' filter

* add changelog fragment

* fail filter uses default message on Undefined or emptystring

makes it slightly easier to use the default message:
```diff
- "{{ None | fail }}"
+ "{{ '' | fail }}"
```

and the user sees a slightly more relevant message
if the message itself is undefined:

```diff
- The error was: {{ failmsg | fail }}: 'failmsg' is undefined
+ The error was: {{ failmsg | fail }}: Mandatory variable has not been overridden
```

* rebuild as the builtin `Undefined`

* harmonise `hint` parameter for make_undefined with jinja

* use code block for documentation item

[ref](https://github.com/ansible/ansible/pull/75435#discussion_r707661035)

* rename to `undef` to expose less Python into the Jinja

[ref](https://github.com/ansible/ansible/pull/75435#pullrequestreview-757799031)

* explicitly instantiate undefined value now that it's possible

see I knew we would break something with reflection

* preserve test coverage of undefined variable

Co-authored-by: Matt Davis <nitzmahone@users.noreply.github.com>

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