Add merged cells support with simple tests

[zackteo]

No description provided.

[codecov-io]

Codecov Report

Merging #42 (ebb9717) into develop (8088a68) will increase coverage by 0.48%.
The diff coverage is 98.52%.

@@             Coverage Diff             @@
##           develop      #42      +/-   ##
===========================================
+ Coverage    98.60%   99.09%   +0.48%     
===========================================
  Files            9        9              
  Lines          502      552      +50     
  Branches         5        3       -2     
===========================================
+ Hits           495      547      +52     
  Misses           2        2              
+ Partials         5        3       -2     

Impacted Files	Coverage Δ
src/zero_one/fxl/write_xlsx.clj	99.31% <95.23%> (-0.69%)	⬇️
src/zero_one/fxl/read_xlsx.clj	96.46% <100.00%> (+1.11%)	⬆️
src/zero_one/fxl/specs.clj	100.00% <100.00%> (+6.00%)	⬆️

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