feat: Add buffer_mut method on Frame

[kdheepak]

current_buffer_mut already exists on Terminal. It would be nice to expose the same thing to Frame as well.

[codecov]

Codecov Report

Merging #548 (1a16fc4) into main (11076d0) will decrease coverage by 0.03%.
Report is 3 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #548      +/-   ##
==========================================
- Coverage   89.65%   89.63%   -0.03%     
==========================================
  Files          41       41              
  Lines       11437    11440       +3     
==========================================
  Hits        10254    10254              
- Misses       1183     1186       +3     

Files	Coverage Δ
src/terminal.rs	57.79% <0.00%> (-0.67%)	⬇️

[joshka]

I think this is a good idea - as it enables working with the frame in sub functions (e.g. things like components).
Let's wait for a second review by one of the other maintainers on this though.