feat(all form-core): add FieldForm in form-core export file

[GuillaumeKESTEMAN]

Related issue

#1090

Description of the issue

I followed this suggested correction : #1090 (comment)

Person(s) for reviewing proposed changes

@samuel-gomez @MartinWeb

Important

Before creating a pull request run unit tests

$ npm test

# watch for changes
$ npm test -- --watch

# For a specific file (e.g., in packages/context/__tests__/command.test.js)
$ npm test -- --watch packages/action