Add experimental JavaScript support when libnode is available

zeek.on('zeek_init', () => {
        console.log('Hello, Zeek!');
    });

For interaction with external systems and HTTP APIs, JavaScript and the
Node.js ecosystem beat Zeek script. Make it more easily accessible by
including ZeekJS with Zeek directly.

When a recent enough libnode version is found on the build system, ZeekJS is
added as a builtin plugin. This behavior can be disabled via
``--disable-javascript``. Linux distributions providing such a package are
Ubuntu (22.10) and Debian (testing/bookworm) as libnode-dev.
Fedora provides it as nodejs-devel.

This plugin takes over loading of .js or .cjs files. When no such files
are provided to Zeek, Node and the V8 engine are not initialized and
should not get into the way.

This should be considered experimental.

testing/scripts/have-javascript

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+if grep -q "ZEEK_HAVE_JAVASCRIPT:INTERNAL=yes" "${BUILD}"/CMakeCache.txt; then
+    exit 0
+fi
+
+exit 1