fix(build): fix schema generation issues

[AThePeanut4]

This fixes a few issues relating to the CI json schema generation.

• befcc1f fixes bug: rust-analyzer schema removed #60, and also fixes several other servers that previously had an empty properties in the schema. This happened because c.properties is a table, which means vim.list_extend doesn't do anything. properties should also always be an object and not an array, so vim.empty_dict() is now being used to prevent outputting [] when properties is empty.
• 84e19f4 embeds index.json rather than downloading it every time. As mentioned in feature: update schema source #54, the gist is no longer maintained, so there's no reason to download it anymore. In the future this should probably be sourced from elsewhere.
• 2a911fd fixes the URL for Volar.
• 6dfd503 is just me running the build step locally. If I shouldn't have done this I'm happy to revert it.

Fixes #52
Fixes #60