feat: custom values and errors

[logaretm]

What

by default, the validation schema ignores errors for non-existent fields, this can be often mistaken for bugs and may not be very intuitive for new users.

This was by design initially to support a use-case of "multi-step" forms, where a schema may partially exist.

However, I have thought about this more and I think this behavior should be customizable.

How

We can introduce a concept of virtual field, basically, the form API would allow you to create custom errors or values.

These fields will be also created if they exist in the schema but didn't have an associated field rendered. This could be a breaking change for some, but I think going forward the schema should be treated as a strict source of truth for the fields that should be rendered.

The use case that would be broken by this behavior is "multi-step" forms, since before this PR a valid partial is considered a valid submission which is problematic more often than not.

For "multi-step" forms we can let it be handled userland, which means you will have to utilize either computed schemas or craft conditional ones to validate each step as its own schema. The official examples will be updated to handle this.

closes #3062

[codecov-commenter]

Codecov Report

Merging #3305 (43b0df6) into main (c915442) will increase coverage by 0.02%.
The diff coverage is 98.49%.

@@            Coverage Diff             @@
##             main    #3305      +/-   ##
==========================================
+ Coverage   96.66%   96.69%   +0.02%     
==========================================
  Files          65       65              
  Lines        1531     1574      +43     
  Branches      371      380       +9     
==========================================
+ Hits         1480     1522      +42     
- Misses         50       51       +1     
  Partials        1        1              

Impacted Files	Coverage Δ
packages/vee-validate/src/Field.ts	100.00% <ø> (ø)
packages/vee-validate/src/validate.ts	96.96% <96.55%> (-0.22%)	⬇️
packages/vee-validate/src/useForm.ts	98.15% <98.75%> (+0.10%)	⬆️
packages/vee-validate/src/useField.ts	98.29% <100.00%> (+<0.01%)	⬆️
packages/vee-validate/src/useIsFieldDirty.ts	100.00% <100.00%> (ø)
packages/vee-validate/src/useIsFieldTouched.ts	100.00% <100.00%> (ø)
packages/vee-validate/src/useIsFieldValid.ts	100.00% <100.00%> (ø)
packages/vee-validate/src/useValidateField.ts	100.00% <100.00%> (ø)
packages/vee-validate/src/useValidateForm.ts	100.00% <100.00%> (ø)

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