feat(components): [form] FormContext adds the getField method

[cloudcome]

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Description

🤖[deprecated] Generated by Copilot at 36b5da1

This pull request enhances the form component and its items by adding a getField function to the parent form and a fieldValue property to the child items. These changes enable easier access and validation of the form values.

Related Issue

Fixes #___.

Explanation of Changes

🤖[deprecated] Generated by Copilot at 36b5da1

• Add fieldValue property to form-item.vue component to get the value of the form item based on its prop and the parent form's modelValue (link)
• Add getField function to form.vue component to get the FormItemContext object of a child form item by its prop from the fields array (link)
• Expose getField function to child form items through the FormContext object returned by the setup function of form.vue and the useForm hook (link)
• Add getField function to the FormContext interface and fieldValue property to the FormItemContext interface in types.ts to define the types of the new properties and methods (link, link)

[pull-request-triage]

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[github-actions]

Hello @cloudcome, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

[ryuhangyeong]

Can you explain your use case?

[cloudcome]

@ryuhangyeong element-plus playground

[cloudcome]

@btea 关注一下

[cloudcome]

@ryuhangyeong element-plus playground

@btea @ryuhangyeong 我们内部将 el-form 封装为 my-form，将 el-form-item 封装为 my-form-item 以便更好的接入内部的表单验证系统，它比内置的 async-validator 要更加的本地化，但在表单验证过程中却需要用到表单和字段元信息。这样的话，更多三方开发者可以更好的扩展 el-form 和 el-form-item。

We internally encapsulate el-form as my-form and el-form-item as my-form-item to better connect to the internal form validation system. It is more localized than the built-in async-validator. However, form and field meta-information is required during form validation. In this way, more third-party developers can better extend el-form and el-form-item.

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=14930
Please comment the example via this playground if needed.

[ryuhangyeong]

@btea @chenxch

Can I ask you to review it

[btea]

@cloudcome Sorry for replying so late. This modification looks good.

[cloudcome]

@cloudcome Sorry for replying so late. This modification looks good.

@btea 期待这个 pr 能够被合并。