This commit moves the existing Spring MVC Mustache support to its own
`servlet` package and adds a new one under `reactive` for the WebFlux
web applications.
New `MustacheView` and `MustacheViewResolver` types resolve and render
Mustache views for WebFlux applications.
Since this templating engine is now supported by two flavors of Spring
web apps, the `spring-boot-starter-mustache` does not depend anymore on
the `spring-boot-starter-web` one: it's up to the developer to add the
relevant starter `web` or `webflux` to their application.
Fixes gh-8648
7e77e648bf