feat: make `IsOptional` conditional

[ruscon]

Description

This is a modified copy of the old PR #196.

Checklist

• [x] the pull request title describes what this PR does (not a vague title like Update index.md)
• [x] the pull request targets the default branch of the repository (develop)
• [x] the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• [x] tests are added for the changes I made (if any source code was modified)
• [x] documentation added or updated
• [x] I have run the project locally and verified that there are no errors

Fixes

fixes #651

[yitzchak-ben-ezra-ecoplant]

This is stuck for too much time.. can save a lot effort of writing custom validators which not always work..

[aqemi]

Please merge this pr

[yitzchak-ben-ezra-ecoplant]

this repo is not maintained, I ended up copying the relevant code to my project it worked great

[NoNameProvided]

Please rebase to review.

[Clashsoft]

What is the difference to ValidateIf?

[yitzchak-ben-ezra-ecoplant]

@Clashsoft this is the difference: ValidateIf - ignore all validations if condition is met IsOptional - allows a field not to be specified when condition is met. but if the field is specified (even if condition is met!): all validations will be checked!!

[joaofranciscoguarda]

Please, aprove this PR! It's must needed

[armansujoyan]

@NoNameProvided can we merge this one? It is very useful validator and people use different workarounds to use it. I am sure community will be thankful for this one 🙌

[braaar]

See my recent comment in #1721