feat: resolve assets in preview

[ktsn]

This is naive implementation that handling assets. The user can preview assets (images) which are specified on src attribute on <img> elements.

I will also support url(...) function in style blocks.

[codecov]

Codecov Report

Merging #31 into master will increase coverage by <.01%.
The diff coverage is 52.85%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   64.47%   64.48%   +<.01%     
==========================================
  Files          44       45       +1     
  Lines        1430     1481      +51     
  Branches      331      334       +3     
==========================================
+ Hits          922      955      +33     
- Misses        493      512      +19     
+ Partials       15       14       -1

Impacted Files	Coverage Δ
src/message/bus.ts	0% <0%> (ø)	⬆️
src/vue-designer.ts	0% <0%> (ø)	⬆️
src/parser/vue-file.ts	0% <0%> (ø)	⬆️
src/server/main.ts	0% <0%> (ø)	⬆️
src/asset-resolver.ts	100% <100%> (ø)
src/parser/template/manipulate.ts	97.29% <100%> (+0.52%)	⬆️
src/parser/style/manipulate.ts	100% <100%> (+9.52%)	⬆️
src/utils.ts	87.5% <80%> (-1.98%)	⬇️

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