Modified Logic to check for optional excerpt

[Queen-codes]

Fixes

• Fixes [Feature] Add Optional Excerpt Field to Blog Post Format #840 by @Queen-codes

Description

This PR modifies the logic that truncates the first 100 characters of the blog post to serve as the excerpt which checks to see if the author provided an excerpt that can be used. If an excerpt is provided, it is used, if not, it falls to the default behaviour.

Technical details

I modified the logic in posts.html, added the excerpt field to the contents.lr file of the write a blog page, and added an excerpt field to the blog-post.ini file to ensure the excerpt field was read correctly.

Tests

To test that the logic worked, I added a dummy blog post to the entries folder and an excerpt field in the contents.lr file.

Screenshots

The added dummy post for testing

The preview it provided

Write a blog post page to show excerpt field in the format provided

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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