[comp: form] 期望表单支持自动设置子项label的长度

[danranVm]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

一般场景下，设计师要求一个表单的标签长度，是表单子项最长标签的长度；
目前不支持自动计算，需要手动指定长度，不够智能；

What does the proposed API look like?

<IxForm labelCol="auto" />

[idux-bot]

Translation of this issue:

[Comp: Form] Expect the form of the automatic settings of the sub -item Label

• [] I have searched the [isSues] (https://github.com/iduxfe/idux/issues) of this repository and believe that this is not a duplicate.

What proplem does this feature solver?

In general, the designer requires the label length of a form, which is the longest label length of the form item;
At present, automatic calculations are not supported, and the length is required manually, which is not intelligent enough;

What does the proposed api look like?

<IxForm labelCol="auto" />