Expressions evaluation fixes

[alexbruy]

Fixes various issues with expressions evaluation:

• do not evaluate expressions when digitizing new feature, to avoid issues when they are re-evaluated in the attribute controller (fix Incorrect default value for new features in some cases #2331)
• re-calculate default values if feature geometry was changed (fix x coordinates in test project are not being updated properly  #2352)
• use correct variant type when setting field values (fix The calculation of the sum of two fields is not correct #1956)
• add form scope to the expression context to support expressions which use form variables (fix Enumeration by expresion doesn't works #2371)
• write attrbute values as is (fix Position variables not updated when editing a point #1756)

[tomasMizera]

It probably fixes #1756 too, right @alexbruy ?

[tomasMizera]

Really cool! 🚀 Great job @alexbruy in finding all of these scenarios

[tomasMizera]

Thanks! We now need to wait for some testing from @jozef-budac or @PatrikMizera

[tomasMizera]

Verified on test projects! 🎉