Rework Gradle build

This commit introduces a _convention plugin_ for RIFE2 support.
It provides a number of advantages:

- the build logic is clearly separated from the build script,
which now only contains user-specific configuration, like the
framework version or how to configure test logging
- it fixes a number of issues like hardcoded dependencies
(`dependsOn` is in general a mistake)
- it removes the need to resolve the agent path eagerly
- it makes the agent configurable
- it clearly separates the user classpath from the RIFE
classpath (both for precompiling templates and for the agent)
- template compilation is cached
- it avoids the abuse of `src/main/resources` to put
templates and uses a dedicated directory instead, which
removes the need for exclusions

In addition, this should make it relatively straightforward
to convert the convention plugin into a proper RIFE2 plugin,
by extracting the code in `build-logic` into its own
repository then publishing to the Gradle plugin portal.

build-logic/build.gradle.kts

@@ -0,0 +1,20 @@
+plugins {
+    `java-gradle-plugin`
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    gradleApi()
+}
+
+gradlePlugin {
+    plugins {
+        create("rife2") {
+            id = "com.uwyn.rife2"
+            implementationClass = "com.uwyn.rife2.gradle.Rife2Plugin"
+        }
+    }
+}