feat: add pkl bom module

[eddie4941]

This adds a module that will publish a bom file that will help consumers
align versions for all pkl artifacts using the single bom.

[eddie4941]

This can be tested by first building the bom pom.xml:

./gradlew :pkl-bom:clean :pkl-bom:generatePomFileForLibraryPublication

and then checking the list of dependencies via:

 grep '<dependency>' -A 4 pkl-bom/build/publications/library/pom-default.xml

which should produce a list of all the included dependencies like:

   <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-cli</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-codegen-java</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-codegen-kotlin</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-commons</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-commons-cli</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-commons-test</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-config-java</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-config-java-all</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-config-kotlin</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-core</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-doc</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-executor</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-formatter</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-gradle</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-parser</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-server</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>
      <dependency>
        <groupId>org.pkl-lang</groupId>
        <artifactId>pkl-tools</artifactId>
        <version>0.31.0-SNAPSHOT</version>
      </dependency>

[bioball]

Thanks, generally looks good! Left a few comments to address.

[bioball]

LGTM