Add task step in to pipeline diagram

[evidolob]

Resolve #352

Demo:

Screen.Recording.2021-04-29.at.17.05.42.mov

[codecov-commenter]

Codecov Report

Merging #560 (060cf29) into master (148e2a1) will decrease coverage by 0.16%.
The diff coverage is 47.50%.

❗ Current head 060cf29 differs from pull request most recent head 9d54ec6. Consider uploading reports for the commit 9d54ec6 to get more accurate results

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
- Coverage   67.28%   67.12%   -0.17%     
==========================================
  Files          96       97       +1     
  Lines        6053     6102      +49     
  Branches     1052     1063      +11     
==========================================
+ Hits         4073     4096      +23     
- Misses       1980     2006      +26     

Impacted Files	Coverage Δ
src/cli-command.ts	65.95% <0.00%> (-0.71%)	⬇️
src/pipeline/preview.ts	19.67% <7.14%> (-0.33%)	⬇️
src/tkn.ts	75.20% <22.22%> (+0.06%)	⬆️
src/humanizer.ts	33.33% <33.33%> (ø)
src/pipeline/pipeline-graph.ts	65.94% <55.55%> (-3.89%)	⬇️
src/pipeline/pipeline-preview.ts	72.91% <100.00%> (ø)
src/tree-view/pipelinerun-node.ts	69.44% <100.00%> (+0.87%)	⬆️
src/tree-view/task-run-node.ts	42.85% <100.00%> (+0.92%)	⬆️
src/util/watchResources.ts	78.00% <100.00%> (ø)
src/yaml-support/tkn-yaml.ts	93.08% <100.00%> (+0.11%)	⬆️
... and 2 more

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