fix(plugin): Copy command from Context Menu should work with numbers

[ghiscoding]

• when trying the feature on a number, I found out that the numbers weren't copied because previous code was calling string methods on every call but we shouldn't on types number/null/undefined

[codecov]

Codecov Report

Merging #575 (9d36491) into master (b4a889f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #575   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          230       230           
  Lines        15775     15776    +1     
  Branches      5089      5089           
=========================================
+ Hits         15775     15776    +1     

Impacted Files	Coverage Δ
packages/common/src/extensions/slickContextMenu.ts	100.00% <100.00%> (ø)
...nilla-bundle/src/salesforce-vanilla-grid-bundle.ts
packages/salesforce-vanilla-bundle/src/index.ts
...s/vanilla-force-bundle/src/vanilla-force-bundle.ts	100.00% <0.00%> (ø)
packages/vanilla-force-bundle/src/index.ts	100.00% <0.00%> (ø)

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