Use default pillar_shaft() method for numbers

[krlmlr]

This gives underlines for every other block of three digits, and all the other details of default pillar formatting.

Working on making formatting customizable, see https://pillar.r-lib.org/dev/articles/numbers.html for a primer. When merged, units will also benefit.

CC @lionel-.

Includes part of #274.

[codecov]

Codecov Report

Merging #273 (7ac8a80) into master (3e451b3) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   94.29%   94.40%   +0.10%     
==========================================
  Files          18       18              
  Lines         877      876       -1     
==========================================
  Hits          827      827              
+ Misses         50       49       -1     

Impacted Files	Coverage Δ
R/conversion.R	88.23% <ø> (ø)
R/tidyverse.R	96.42% <100.00%> (+3.32%)	⬆️

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[Enchufa2]

LGTM. Thanks!