when mounting dynamic components, set ref if required

[Conduitry]

Refs to dynamic components weren't being set during mounting, only during updates. This fixes that and adds a test.

cc @heardy

[codecov-io]

Codecov Report

Merging #1280 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1280      +/-   ##
==========================================
+ Coverage   91.51%   91.57%   +0.05%     
==========================================
  Files         121      121              
  Lines        4326     4330       +4     
  Branches     1361     1363       +2     
==========================================
+ Hits         3959     3965       +6     
  Misses        153      153              
+ Partials      214      212       -2

Impacted Files	Coverage Δ
src/generators/nodes/Component.ts	95.93% <100%> (+1.04%)	⬆️
src/shared/utils.js	100% <0%> (ø)	⬆️
src/generators/dom/index.ts	96.06% <0%> (+0.02%)	⬆️

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[Conduitry]

I realized I'm not actually sure about addLine vs addBlock. Is this purely an aesthetic thing with whitespace in the generated code? I'm thinking probably this should be addBlock anyway though.

[Rich-Harris]

Is this purely an aesthetic thing with whitespace in the generated code?

yeah. it's used inconsistently, and often results in weird spacing to be honest. there's probably a better way