Enable rockspec

[sankantsu]

Should resolve 3rd#191

This PR introduce following changes:

• Rename *.rockspec.disabled -> *.rockspec to enable lazy.nvim can detect rockspec.
• Simply use default branch URL for src.url (no need to manual update).
• Update install instruction for lazy.nvim >= v11.

Re-enabling .rockspec file benefits lazy.nvim users, enabling automic build of plugin and magick rocks. It eliminates the need to manually change package.path.

References:

• Lazy.nvim
  • Lazy.nvim: https://lazy.folke.io/packages
  • Lazy.nvim issue: luarocks needs lua 5.1 folke/lazy.nvim#1570
• Luarocks
  • Creating a rock: https://github.com/luarocks/luarocks/wiki/Creating-a-rock
  • Rockspec format: https://github.com/luarocks/luarocks/wiki/Rockspec-format

Disclaimer:
I'm not an expert of Lua and Luarocks.