Add readonly type to renderers prop in JsonForms.vue

[rokiyama]

Fix #1859

This PR adds the readonly type to renderers props in JsonForms.vue.

According to the README of jsonforms/vue, renderers may be the value of Object.freeze. So it might be better that renderers also accept readonly types.

[CLAassistant]

All committers have signed the CLA.

[sdirix]

Hi @rokiyama, thank you very much for the contribution! ❤️

Your PR solves the issue for the renderers but it would be even nicer if it could be solved for the remaining registries too.

I would like to suggest introducing a MaybeReadonly type, e.g.

type MaybeReadonly<T> = T | Readonly<T>;

and using that for the props renderers, cells and uischemas

[coveralls]

Coverage remained the same at 84.242% when pulling 5b98584 on rokiyama:fix-renderers-type-error-in-vue into 075bbc4 on eclipsesource:master.

[rokiyama]

@sdirix Sorry for my late reply. I read your commit 5b98584 , it would help me if this change is merged.

[sdirix]

Tested and fixed the type issue when using the Volar VS Code extension