feat: only emit value when control is valid

[lothern]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngneat/reactive-forms/blob/master/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

https://github.com/ngneat/reactive-forms/issues/182

Issue Number: 182

What is the new behavior?

New property called validValue$ that only emits when a value is valid.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[bolt-new-by-stackblitz[bot]]

Run & review this pull request in StackBlitz Codeflow.