Add SPDX header

[AndyScherzinger]

• Added SPDX info to all files
• Added GH action for compliance check
• Added REUSE compliance status badge to README.md

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 23.59%. Comparing base (040276a) to head (9196525).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/App.vue	0.00%	1 Missing ⚠️
...alendarList/CalendarListItemLoadingPlaceholder.vue	0.00%	1 Missing ⚠️
...omponents/Editor/Repeat/RepeatExceptionWarning.vue	0.00%	1 Missing ⚠️
src/components/Editor/Repeat/RepeatForkWarning.vue	0.00%	1 Missing ⚠️
...ponents/Editor/Repeat/RepeatUnsupportedWarning.vue	0.00%	1 Missing ⚠️
src/components/Popover/PopoverLoadingIndicator.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6046   +/-   ##
=========================================
  Coverage     23.59%   23.59%           
  Complexity      457      457           
=========================================
  Files           246      246           
  Lines         11714    11714           
  Branches       2150     2140   -10     
=========================================
  Hits           2764     2764           
- Misses         8635     8636    +1     
+ Partials        315      314    -1     

Flag	Coverage Δ
javascript	15.18% <0.00%> (ø)
php	58.61% <ø> (ø)

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