Add Signal::modify to mutate a value

[lukechu10]

No description provided.

[codecov]

Codecov Report

Merging #399 (fe931e4) into master (75850ef) will increase coverage by 0.08%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   63.26%   63.35%   +0.08%     
==========================================
  Files          48       48              
  Lines        7786     7812      +26     
==========================================
+ Hits         4926     4949      +23     
- Misses       2860     2863       +3     

Impacted Files	Coverage Δ
packages/sycamore-reactive/src/signal.rs	87.02% <88.46%> (+0.11%)	⬆️

---

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 75850ef...fe931e4. Read the comment docs.