Add LiveView formatter

[SimonLab]

• Use formatter to format .heex file
• Add Github action to check the code is formatted on CI
• Run mix format

see: #183

[codecov[bot]]

Codecov Report

Merging #184 (636b9c7) into main (4da9388) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #184   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          219       222    +3     
=========================================
+ Hits           219       222    +3     

Impacted Files	Coverage Δ
lib/app/tag.ex	100.00% <0.00%> (ø)
lib/app/item.ex	100.00% <0.00%> (ø)
lib/app_web/live/app_live.ex	100.00% <0.00%> (ø)
lib/app_web/controllers/auth_controller.ex	100.00% <0.00%> (ø)

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[SimonLab]

the Github workflow is running the check using mix format --check-formatted, however the LiveView formatter is ignored:

So I think templates .heex are not checked on CI at the moment. I'm not sure why, maybe an old version of Phoenix.LiveView is still used with Github actions?

However this PR should still allow us to format the code and templates on our machines. I'll keep an eye on the next PRs to see if the LiveView formatter is found