Ignore BV keys when auto-discovering table fields

[robcresswell]

Bootstrap-Vue has some extra keys that can be provided to enhance table
functionality. These should be explicitly ignored when automatically
discovering the fields for a table, when the fields prop is not
provided.

The reason for explicitly ignoring the keys, rather than blanket
ignoring all keys beginning with "_" is to minimise any possible
collisions with user provided data in that field.

Fixes #1635

[codecov]

Codecov Report

Merging #1641 into dev will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #1641      +/-   ##
==========================================
+ Coverage   60.64%   60.78%   +0.13%     
==========================================
  Files         153      153              
  Lines        2869     2871       +2     
  Branches      788      789       +1     
==========================================
+ Hits         1740     1745       +5     
+ Misses        809      806       -3     
  Partials      320      320

Impacted Files	Coverage Δ
src/components/table/table.js	72.54% <100%> (+1.16%)	⬆️

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