fix: add icons to EditorTree and optimize its style

[kiwiwong]

Description

Add icons to EditorTree and optimize its style

Fixes #531

[codecov]

Codecov Report

Merging #656 (8156a77) into main (4a3470f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #656   +/-   ##
=======================================
  Coverage   89.34%   89.34%           
=======================================
  Files         185      185           
  Lines        5614     5614           
  Branches     1332     1332           
=======================================
  Hits         5016     5016           
  Misses        590      590           
  Partials        8        8           

Impacted Files	Coverage Δ
src/workbench/sidebar/explore/editorTree.tsx	100.00% <100.00%> (ø)

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[wewoor]

+1