rockspec files are used by [Luarocks](https://luarocks.org/), the
most prominent package manager for Lua. Despite the different file
extension, these files are actually Lua files and should be syntax
highlighted as such.
For what it is worth, Neovim also does the same thing that I am doing in
this commit. They recognize both ".lua" and ".rockspec" as being Lua
files (and no other extensions, as far as I know).
This fixes a bug in how the Lua scripts handle new comment lines.
Currently if we have a comment that is indented, when we add a new line
it inserts the `--` prefix before the automatic indentation.
```
--ABC
-- XYZ
```
After the fix, it correctly inserts the comment prefix after the
indentation:
```
--ABC
--XYZ
```
The solution I used is inspired by the ruby.kak script.
Current solution makes it difficult to use common Lua practices of having one-liner if statements and using anonymous functions.
New solution prevents auto-indentation and end insertion, if the previous line contains an "end" keyword.
It does not attempt to match each structure with corresponding end, since using multiple end keywords in single line is a very rare occurance in Lua.
This commit allows using the <semicolon> expansion in commands, instead
of `\;`.
It makes commands look more elegant, and prevents new-comers from
falling into the trap of using <a-;> without escaping the semicolon.
