feat(signals): initial setup

[markostanimirovic]

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• Tests for the changes have been added (for bug fixes / features)
• Documentation has 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)
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Closes #3992

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[timdeschryver]

Do we also need to exclude signals here @brandonroberts ?
The rest LGTM 🚀

https://github.com/markostanimirovic/platform/blob/0fcf1371534094f43d92b1077ad5bd21d0e25a95/build/config.ts#L23-L24

[markostanimirovic]

Good point @timdeschryver! I excluded the signals package from releasing flow since it will be released separately in the beginning.