Rewrite [[, ]], [], ][ implementation.

[Oberon00]

The new semantic is according to
http://learnvimscriptthehardway.stevelosh.com/chapters/50.html:

• ]] jumps to the start of the next function,
• ]] to the start of the previous function,
• ][ to the end of the next function,
• [] the end of the previous function.

[idbrii]

This is much more consistent than the current implementation (which jumps to one function defined in a function but not to the second function defined in the same top-level function).

However, what I really want is to jump to the function in the same scope (if I'm in a subfunction, jump to the next subfunction. if in a top-level, skip past subfunctions). I attempted this in idbrii/vim-lua-ftplugin@0e12923 and a7c3606, but it doesn't work well.

Regardless, in my brief testing this PR is an improvement on the status quo!