Allow a specific version of Encore to be installed via the install script

[dcitron]

For self-hosted Encore instances, the https://encore.dev/install.sh should provide a way to install a specific version of Encore.

The easiest approach for us would be if there was an option for the script to read the package.json and pull the matching version.

But if that's not possible, we can write our own wrapper script.

See the conversation here for more context: https://discord.com/channels/814482502336905216/1382718812314537995

See #1967 for some additional background.

[elirov]

This is a critical requirement of any dependency library. I'm surprised it's not supported.