Add preview for pipeline tasks

[evidolob]

Fix: #150

This PR add Open Pipeline preview to the Side command in editor title bar, which open pipeline preview webview.

In webview used cytoscape with dagre plugin (to render DAG tree), JS lib to render pipeline task execution diagram

Current demo:

First diagram on the demo is equal to diagram from tekton documentation

Things to do:

• Implement webview persistence, now all content lost when preview tab switched
• Implement support of vscode theme switching
• Update diagram on pipeline editing

[codecov-io]

Codecov Report

Merging #160 into master will increase coverage by 1.59%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
+ Coverage   71.68%   73.27%   +1.59%     
==========================================
  Files          33       33              
  Lines        1865     1860       -5     
  Branches      291      291              
==========================================
+ Hits         1337     1363      +26     
+ Misses        528      497      -31

Impacted Files	Coverage Δ
src/pipeline/preview-manager.ts	26.66% <ø> (+3.8%)	⬆️
src/pipeline/pipeline-preview.ts	86.36% <100%> (+18.18%)	⬆️
src/extension.ts	88% <100%> (ø)	⬆️
src/pipeline/customTektonExplorer.ts	61.29% <50%> (ø)	⬆️
src/pipeline/pipelineExplorer.ts	57.77% <50%> (ø)	⬆️
src/pipeline/pipeline-graph.ts	96.42% <0%> (+78.57%)	⬆️

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[siamaksade]

cc @vdemeester @sthaha

[vdemeester]

Nice ! 😻

[evidolob]

Update diagram preview on pipeline change in editor demo:

cc @siamaksade

[evidolob]

Theme switching demo: