Replace `allprojects {}` config with convention plugins

[aSemy]

Currently the project uses allprojects {} to configure all subprojects.

kotlinx-knit/build.gradle.kts

Lines 24 to 27 in c4e77da

	allprojects {
	apply(plugin = "org.jetbrains.kotlin.jvm")
	apply(plugin = "org.gradle.maven-publish")

Using allprojects {} is not recommended by Gradle. Is is for number of reasons which are explained thoroughly elsewhere (1, 2), so I won't detail them again.

One specific issue that Knit encounters is that Jackson is added to all projects, even though this dependency is only required in 'Dokka plugin' and (non-Gradle) Knit project.

kotlinx-knit/build.gradle.kts

Lines 33 to 38 in c4e77da

	dependencies {
	implementation(kotlin("stdlib-jdk8"))
	implementation("com.fasterxml.jackson.module:jackson-module-kotlin:2.11.1")
	implementation("com.fasterxml.jackson.dataformat:jackson-dataformat-xml:2.11.1")
	testImplementation(kotlin("test-junit"))
	}

This can be resolved by creating buildSrc convention plugins that can be applied specifically to subprojects as required, avoiding duplicate configurations.