WIP add support for new icon sets

[imirkin]

PR Details

This adds support for the "new" icon sets (3 triangles, 3 stars, 5 boxes). These were introduced in a later revision of the spec, and thus need different namespaces/etc.

Description

For the new icon sets, we add an x14-namespaced conditional formatting entry + rules. Note that unlike the data bar, these are entirely in the "new" land, so no need to add it in the old location + ext.

Still WIP:

• Strategy for priorities
• GUID generation? The Excel-generated samples have GUIDs, but seem to work without it
• Decoding

Looking for some feedback/suggestions.

Related Issue

#2038

How Has This Been Tested

I added the conditional format with all 3 "new" icon sets (as well as a data bar with solid=true just to make sure I'm not breaking anything existing), and it displayed fine in Excel.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Attention: Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.

Project coverage is 99.19%. Comparing base (8e04909) to head (38c080f).

Files with missing lines	Patch %	Lines
styles.go	94.54%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2042      +/-   ##
==========================================
- Coverage   99.20%   99.19%   -0.01%     
==========================================
  Files          32       32              
  Lines       29975    30017      +42     
==========================================
+ Hits        29737    29776      +39     
- Misses        158      160       +2     
- Partials       80       81       +1     

Flag	Coverage Δ
unittests	99.19% <94.54%> (-0.01%)	⬇️

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