Font size sync

[simc]

Allows to sync multiple AutoSizeTexts.

[codecov]

Codecov Report

Merging #6 into master will decrease coverage by 1.91%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   96.87%   94.95%   -1.92%     
==========================================
  Files           2        3       +1     
  Lines          64      119      +55     
==========================================
+ Hits           62      113      +51     
- Misses          2        6       +4

Impacted Files	Coverage Δ
lib/src/auto_size_text.dart	97.61% <100%> (+1.06%)	⬆️
lib/src/auto_size_group.dart	86.2% <86.2%> (ø)

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