Textual: Allow constraining arc a/b points

[adamchalmers]

Previously you could only fix an arc's center.
Now you can fix an arc's A or B point (either end).

[codspeed-hq]

CodSpeed Performance Report

Merging #156 will not alter performance

_{Comparing achalmers/parser-a-and-b (df2c6ad) with main (a8fc3c3)}

Summary

✅ 13 untouched