[Templating] Allows "template" as parameter name

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | n/a

Allows "template" as parameter name but restrict usage of "__template__" and "parameters" to avoid any conflict.

Before the patch, if `$template` is not unset and a parameter is named "template", it is not extracted to the template. The `$template` contains the `Templating\Storage` instance instead of the use value.