Add the ability to change dropdown width

[DucNg]

Changing dropdown size only using css was not easily achievable since vti__dropdown-list was placed inside vti__dropdown making width: 100% not working properly.

I figured out adding a "dropdown width" option would be a much easier and cleaner solution.

Is that okay with you?

[iamstevendao]

@DucNg Thanks for your help! it's available in v5.9.0

[DucNg]

Thank you so much!

[gnuletik]

For anyone upgrading: this PR is a breaking change if you have the dropdown-options prop already defined.
The default width value is only used if the prop is not defined. So, the dropdown width may break.