fix: add rel to fontTable

[YousefED]

closes #2521

After this change, I think custom embedded fonts are working.

One feature that still seems to be missing is that docxjs only allows for "regular" fonts to be embedded, i.e. afaik it's not possible to create embeds of different variants like this:

<w:embedRegular r:id="rId3" w:fontKey="{ED927D51-56FC-1C41-961F-307024631508}" />
        <w:embedBold r:id="rId4" w:fontKey="{D6B4F25E-1ACA-814F-AD3C-35D85CD7A6D5}" />
        <w:embedItalic r:id="rId5" w:fontKey="{E5B26720-9505-4B41-AB02-5860705B4C01}" />

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2381ba8) to head (5476a57).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2800   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          281       281           
  Lines        14093     14099    +6     
  Branches      1381      1381           
=========================================
+ Hits         14093     14099    +6     

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