Build the Silver runtime with maven instead of ant

[krame505]

There are a few reasons why we should probably do this:

• Currently we just keep a pile of jar dependencies for building the runtime hosted on our website, including CopperCompiler.jar, CopperRuntime.jar and commonmark-0.17.1.jar. For a number of reasons, this is Not Great.
• To build the Silver langserver with maven, we currently install the runtime jars to the local maven repo. This would be a bit nicer if the runtime was already a maven project.
• This would also simplify splitting out the language-agnostic bits of the Silver language server into a library for building LSP server implementations for languages implemented in Silver.
• It would make setting up VS Code or other IDEs for working on the Silver runtime a bit nicer, given an existing Maven project.

One prerequisite of this is probably getting the Copper artifacts published in the maven central repository. Otherwise Silver users would need to clone Copper and run mvn install in order to run ./deep-rebuild.

The Silver runtime maven project would still depend on the generated java sources from silver:core, and compiling silver:core requires a pre-built runtime, so we wouldn't easily be able to distribute the Silver runtime on the maven central repo. We could still host the runtime jar on our website alongside the compiler jar and update it through Jenkins - but these would be the only two jars we would need to host.