feat: add validate cron from string

[joserafaelSH]

feat(cronValidator): checks whether an expression made up of digits and allowed characters with 6 fields is valid. The input is a string that may or may not contain the start and end parentheses and the return is a boolean confirming whether the cron is valid or not.

Checklist

• PR contains only changes related; no stray files, etc.
• [] README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

Attention: 53 lines in your changes are missing coverage. Please review.

Comparison is base (b958bd7) 99.95% compared to head (a3909c5) 97.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2306      +/-   ##
==========================================
- Coverage   99.95%   97.92%   -2.04%     
==========================================
  Files         107      108       +1     
  Lines        2454     2604     +150     
  Branches      619      676      +57     
==========================================
+ Hits         2453     2550      +97     
- Misses          0       36      +36     
- Partials        1       18      +17     

Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)
src/lib/isCron.js	65.58% <65.58%> (ø)

... and 2 files with indirect coverage changes

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