support em/ex units in root SVG size

[swhitty]

Fixes a regression that was added in 0.28.0 / #126 where SVGs with width/height specified in em / ex units no longer parsed.

While em / ex units are not resolved correctly at all within swift draw, this PR restores the previous behaviour parsing these units as numeric values.

Resolves #130

[codecov]

Codecov Report

❌ Patch coverage is 97.56098% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.81%. Comparing base (edbf753) to head (837ecdb).

Files with missing lines	Patch %	Lines
DOM/Sources/Parser.XML.SVG.swift	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   89.73%   89.81%   +0.08%     
==========================================
  Files         163      163              
  Lines       12974    13051      +77     
==========================================
+ Hits        11642    11722      +80     
+ Misses       1332     1329       -3     

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