fix: legend sizing issue with no initial legend

[nickofthyme]

Summary

Prevent setting legend width when element width is 0.

Fix 0 width legends in lens

Before

After

Trigger legend to resize (grow/shrink) when legend item count changes

Dynamic charts such as TSBV create dynamic legend content which leaves legends squished when resize is only on initialization.

fixes #367

Adds build:watch script to run elastic-charts in watch mode when symlinked to kibana.

Checklist

• This was checked for cross-browser compatibility, including a check against IE11

Verified in Chrome, Firefox, safari and IE. All look good, though IE seems to not be fully supported rn.

• Unit tests were updated or added to match the most common scenarios
• Each commit follows the convention

[codecov-io]

Codecov Report

Merging #441 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #441   +/-   ##
=======================================
  Coverage   97.88%   97.88%           
=======================================
  Files          41       41           
  Lines        2927     2927           
  Branches      710      692   -18     
=======================================
  Hits         2865     2865           
  Misses         55       55           
  Partials        7        7

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[markov00]

🎉 This PR is included in version 13.5.9 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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