feat(buffer): deprecate Cell::symbol field

[rhysd]

This PR deprecates accessing Cell::symbol field following the discussion at #601.

1. Add deprecation message on Cell::symbol field and getter API for the field
2. Release ratatui v0.24.1 (or v0.24.2 if v0.24.1 doesn't fit)
3. Hide the symbol field
4. Release the change at v0.25.0

This PR does the step 1. assuming to be included in v0.24.1.

[codecov]

Codecov Report

Merging #624 (6a52360) into main (df0eb1f) will increase coverage by 0.0%.
The diff coverage is 88.0%.

@@          Coverage Diff          @@
##            main    #624   +/-   ##
=====================================
  Coverage   90.4%   90.4%           
=====================================
  Files         41      41           
  Lines      11890   11903   +13     
=====================================
+ Hits       10758   10771   +13     
  Misses      1132    1132           

Files	Coverage Δ
src/backend/termion.rs	26.7% <100.0%> (ø)
src/backend/test.rs	97.7% <100.0%> (ø)
src/widgets/canvas/line.rs	99.5% <100.0%> (ø)
src/backend/crossterm.rs	0.0% <0.0%> (ø)
src/backend/termwiz.rs	0.0% <0.0%> (ø)
src/buffer.rs	96.8% <94.4%> (+<0.1%)	⬆️

[orhun]

LGTM

[joshka]

Thanks for the PR