fix: can't uncomment in sexy mode with NERDCompactSexyComs=1 for lua

[kevinhwang91]

reproduce issue:

1. open lua file;
2. run ex let g:NERDCompactSexyComs = 1;
3. enter visual mode and select multiple lines;
4. <leader>cs;
5. <leader>cu;

solve this issue by escaping [ and ].

[alerque]

Thanks for taking the time to contribute this. I'd definitely like to see it fixed. Before I merge however I'd like to understand how this will/won't affect other languages and issues. We've had some uncommenting issues reported before that we've never gotten around to fixing and I wonder how related this is. See e.g. #339, #341, #68, #295, #291, #74, #150, and maybe others.

At the very least this is probably a fix for #150, but lets also determine its impact on other issues.

[kevinhwang91]

I think it's not related above issues, this issue is caused by

nerdcommenter/plugin/NERD_commenter.vim

Lines 2257 to 2259 in 119b99d

	"the right delimiter is on the same line as the last sexyComMarker
	elseif theLine =~# '^[ \t]*' . sexyComMarker . '.*' . right
	let bottom = currentLine

theLine value at the lastline looks like [ lastline ]].
In lua, '^[ \t]*' . sexyComMarker . '.*' . right = '^[ \t]*' . '[' . '.*' . ']]'. Without escaping [ and ], ^[ \t]*[.*]] can't match [ lastline ]].

This PR can't solve other issues but in order to fix the match for left and right.