fix: retain number prompt default

[tjallingt]

The number prompt wants to use the default value if the input was invalid, however after a keypress (and if the default value is truthy) the default gets cleared. This is done so the default value is not rendered in the prompt anymore, see: #719

This PR changes the logic so that instead of clearing the default value it sets a new state called "touched". We can then use this state to hide the default value from the rendered prompt.

Updated the relevant test so it would have failed before.

[codecov]

Codecov Report

Merging #1020 (2bc4808) into master (c4a3c23) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
+ Coverage   93.20%   93.28%   +0.08%     
==========================================
  Files          27       27              
  Lines        1163     1162       -1     
  Branches       25       25              
==========================================
  Hits         1084     1084              
+ Misses         79       78       -1     

Impacted Files	Coverage Δ
packages/inquirer/lib/prompts/base.js	92.15% <100.00%> (ø)
packages/inquirer/lib/prompts/input.js	100.00% <100.00%> (+2.38%)	⬆️

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[SBoudrias]

Thanks!