Shorten the handling of /A and /a page labels

[Snuffleupagus]

I happened to glance at this code, and noticed that using the style-value itself to compute the current character would shorten this code a little bit.

Note that this particular page label format seems to be somewhat rarely used in practice, compared to e.g. the roman numerals format, which probably isn't that strange given its definition: https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf#G11.2096063

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.19%. Comparing base (0f26334) to head (3451fc5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21814      +/-   ##
==========================================
- Coverage   90.20%   90.19%   -0.01%     
==========================================
  Files         264      264              
  Lines       67339    67336       -3     
==========================================
- Hits        60741    60733       -8     
- Misses       6598     6603       +5     

Flag	Coverage Δ
browsertest	66.31% <ø> (-0.03%)	⬇️
fonttest	8.91% <ø> (ø)
integrationtest	69.40% <ø> (+<0.01%)	⬆️
unittest	58.36% <ø> (-0.01%)	⬇️
unittestcli	56.94% <ø> (+0.01%)	⬆️

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[timvandermeij]

Thank you for improving this code!