Support underline style

[tangjingyuan]

Support underline style and Add underline button in toolbar.

[codecov]

Codecov Report

Merging #365 into master will decrease coverage by 0.07%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
- Coverage   91.89%   91.82%   -0.08%     
==========================================
  Files          11       11              
  Lines        1074     1076       +2     
==========================================
+ Hits          987      988       +1     
- Misses         87       88       +1     

Impacted Files	Coverage Δ
packages/notus/lib/src/document/attributes.dart	81.33% <50.00%> (-0.43%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0d69499...68977d8. Read the comment docs.