fix(popup): properly apply winhighlight

[MunifTanjim]

Highlight complex border characters explicitly as FloatBorder. Complex border's Normal should be the same as the actual popup's Normal.

[codecov[bot]]

Codecov Report

Merging #219 (6d51928) into main (7427f97) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
+ Coverage   98.03%   98.07%   +0.04%     
==========================================
  Files          19       19              
  Lines        2288     2286       -2     
==========================================
- Hits         2243     2242       -1     
+ Misses         45       44       -1     

Impacted Files	Coverage Δ
lua/nui/popup/border.lua	97.32% <100.00%> (+0.31%)	:arrow_up:

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