Add Mustache support for Spring WebFlux apps

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

spring-boot-samples/spring-boot-sample-web-mustache/pom.xml

@@ -21,6 +21,10 @@
 	</properties>
 	<dependencies>
 		<!-- Compile -->
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-web</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-mustache</artifactId>