Cell with unit tests, styling, and interfaces

[pottedmeat]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

Adds the Cell widget with styling and unit tests. Adds interfaces used by the Cell properties.

Supports an optional cellRenderer that is passed an item and can return either a widget constructor or the name of a registered widget constructor.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@aaa6a40). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #6   +/-   ##
=========================================
  Coverage          ?   51.72%           
=========================================
  Files             ?        4           
  Lines             ?       29           
  Branches          ?        1           
=========================================
  Hits              ?       15           
  Misses            ?       14           
  Partials          ?        0

Impacted Files	Coverage Δ
src/Cell.ts	100% <100%> (ø)

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