Add body option to gitalk configuration to support custom issue content

[tootscharlie]

PR Checklist

• The changes have been tested (for bug fixes / features).
• Docs in NexT website have been added / updated (for features).

Docs PR: next-theme/theme-next-docs#138

PR Type

• Bugfix.
• Feature.
• Improvement.
• Code style update (e.g. formatting, linting).
• Refactoring (no changes to functionality and APIs).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Custom gitalk issue body is currently not supported

What is the new behavior?

gitalk config add body config item, add custom issue body configuration.

• Link to demo site with this changes: None
• Screenshots with this changes: None

How to use?

In NexT _config.yml:

gitalk:
  // omit other configuration..
  # If you want to customize the content of the initialized Issue, please edit the body parameters. support line breaks via `|`.
  # Sample configuration: ${document.title}` or `${location.href}
  body: |
    **Article title**: [${document.title}](${location.href})
    **Customized content**: This is my customized Issue body in Gitalk. Comments and exchanges are welcome!

[welcome]

Thanks so much for opening your first PR here!

[github-actions]

This pull request contains changes to the configuration file. Please make sure the documentation in NexT website is changed or added.
Please edit relevant source files here: https://github.com/next-theme/theme-next-docs/tree/master/source/docs and create a pull request with the changes here: https://github.com/next-theme/theme-next-docs/pulls

[CLAassistant]

All committers have signed the CLA.

[stevenjoezhang]

new Function poses security risks and may lead to unintended code execution. I'll make some adjustments.

[coveralls]

Pull Request Test Coverage Report for Build 19035283207

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.451%

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Totals
Change from base Build 19025960297:	0.0%
Covered Lines:	400
Relevant Lines:	405

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💛 - Coveralls

[welcome]

Congrats on merging your first pull request here! 🎉 How awesome!

[tootscharlie]

:)