feat: add giscus support

[iju707]

feat: #89

add giscus support.
There are other comment service, so I created it using comment.type.

in hugo.yml

params:
  comment:
    enable: true
    type: giscus
    giscus:
      dataRepo: data-repo parameter
      dataRepoId: data-repo-id parameter
      dataCategory: data-category parameter
      dataCategoryId: data-category-id parameter
      dataMapping: data-mapping parameter (default pathname)
      dataLang: data-lang parameter (default en)

add components/comment partial in all list/single page, except list page in blog.

if some page want to disable comment, use comment: false in front matter.

---
title: test page
comment: false
---

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[imfing]

thanks for the PR, it looks great overall!

I have a few small suggestions on the configuration:

params:
  comment:
    enable: true
    type: giscus

    giscus:
      repo: data-repo parameter
      repoId: data-repo-id parameter
      category: data-category parameter
      categoryId: data-category-id parameter
      mapping: data-mapping parameter (default pathname)

• remove data prefix for the giscus parameters
• remove lang setting, replace with the site language code, so it works with multilingual site

[imfing]

Thanks, merged! We also need to add documentation and example settings in separate PR