[Feature] support pinning dependencies

[NTBBloodbath]

I think it would be very useful to support a lockfile to always install the plugins and their dependencies with the same version if you have to change the Neovim installation either because you installed your dotfiles on another machine or another reason.

Luarocks supports this natively, please take a look at Pinning versions with a lock file.

[mrcjkb]

Perhaps we could incorporate the luarocks lock file into a section in rocks.toml:

[pinned-dependencies]
jsregexp = "0.0.6"

so that users don't have to maintain two files in scm.

[mrcjkb]

Perhaps we could incorporate the luarocks lock file into a section in rocks.toml:

[pinned-dependencies]
jsregexp = "0.0.6"

so that users don't have to maintain two files in scm.

Scrapping this idea, because I'd prefer to keep rocks.toml as clean as possible.