Allow defining custom code/formulas to reuse

[XoMEX]

Describe the problem to be solved

Disclaimer: this issue only exists because the PR template asks for an issue when suggesting a new issue

We regularly use Grist in our department to manage grading of students (previously we used shared spreadsheets, but managing that was a mess, especially the formulas). In this, we have multiple columns that just transform a percentage into a grade. Hence, we paste the same big formula into each of those columns. I (as the de-facto sysadmin of our group) would like to keep the formulas DRY and define them once per document, so they can be easily changed at a central place. Similarly, I'd like to use some variables in the validation fields which I can define globally.

Describe the solution you would like

I would like a document setting where I can define custom functions and constants that I can then use in column formulas and validation.