Lua 5.4 support

[nvs]

A few checks need to be done to ensure that Lua 5.4 works properly. Luacheck 0.23 does not support 5.4 explicitly, and this might present an issue. Anyone using LuaRocks to install dependencies will get Luacheck 0.24, which has taken steps to support 5.4.

[nvs]

A check of some dependencies shows that the following work with Lua 5.4:

• luacheck 0.24
• luafilesystem1.8.0
• lua-stormlib 0.2.3
• compat-5.3 0.8

Probably best to allow installation via Luarocks to assist with dependency tracking, as mentioned in #36. I think I would prefer to install this via a 'local' repository, whatever the terminology is.