Add PosID() Function for Retrieving MeCab's POS ID

[kmurata08]

Dear shogo82148,

I hope this message finds you well. I am writing to propose a new feature for the go-mecab library, which I believe will enhance its functionality.

Feature Description:
The feature I've implemented is the PosID() function, which enables users to retrieve the Part-of-Speech (POS) ID from MeCab. This addition will provide developers with more flexibility and utility when working with MeCab in Go, especially for those dealing with advanced linguistic processing tasks.

Implementation Details:

• The PosID() function accesses MeCab's internal POS ID for each token.
• It returns the POS ID as an integer.

Thank you for considering my contribution. I look forward to your feedback.

Best regards,
kmurata08

[shogo82148]

LGTM

thanks!