[5.x] Enable unselecting fieldtypes for forms

[godismyjudge95]

Sometimes it might be necessary to disable certain fieldtypes for frontend forms - per discord conversion with Marco - https://discord.com/channels/489818810157891584/489818810157891586/1349028284142194699

This PR adds a new method to accomplish that on the FieldtypeRepository and Fieldtype classes: makeUnselectableInForms()

[jasonvarga]

This seems to only allow you to make an unselectable fieldtype unselectable again if you've already made them selectable.

You can't make other normally-selectable fieldtypes unselectable.

e.g. Try to make the toggle fieldtype unselectable.

[godismyjudge95]

This seems to only allow you to make an unselectable fieldtype unselectable again if you've already made them selectable.

You can't make other normally-selectable fieldtypes unselectable.

e.g. Try to make the toggle fieldtype unselectable.

oh oops, I'll fix it thanks for the catch