Implement Tree.Style for setting attributes to nodes

[hackerwins]

What this PR does / why we need it:

Implement Tree.Style for setting attributes to nodes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #549 (c32193b) into main (0ce364b) will decrease coverage by 0.11%.
The diff coverage is 46.87%.

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
- Coverage   50.53%   50.43%   -0.11%     
==========================================
  Files          67       67              
  Lines        6678     6763      +85     
==========================================
+ Hits         3375     3411      +36     
- Misses       2876     2923      +47     
- Partials      427      429       +2     

Impacted Files	Coverage Δ
api/converter/to_pb.go	57.27% <0.00%> (-1.98%)	⬇️
pkg/index/tree.go	53.18% <0.00%> (ø)
api/converter/from_pb.go	49.23% <7.40%> (-2.30%)	⬇️
api/converter/to_bytes.go	72.29% <64.28%> (-2.18%)	⬇️
pkg/document/crdt/tree.go	72.29% <70.96%> (-0.12%)	⬇️
pkg/document/crdt/rht.go	68.25% <100.00%> (+7.46%)	⬆️