use the Ceylon version to choose the version of the language server to run

[jvasileff]

We should support multiple Ceylon versions without the user having to re-install Ceylon version specific versions of the VSCode extension.

• The extension should first detect the version of Ceylon installed/configured on the users system (already done)
• The Ceylon version should be used to lookup the right version of the language server to run
• A new (undocumented?) extension setting should be created to allow overrides of the lookup map

This feature would be very consistent with the "feel" of the extension, as the extension is designed to seamlessly adapt to the user's environment rather than requiring lots of extra settings and installation complexity.

Potential problems:

• Extra bloat shipping multiple versions of dependencies, such as ceylon.markdown and ceylon-ide-common. (The language server itself should be made available on herd)
• Complications with version specific overrides.xml and maven settings.xml.
• Changes to the client will require updates to old server releases.