[Term Entry] Dart List .fold()

[Christian Dinh]

Reason/inspiration (optional)

We would like a new term entry under Dart List for the Dart List .fold() method.
Provide the entry in a markdown file under docs/content/dart/concept/list/terms/fold/fold.md

The entry should
- Have an introductory paragraph about what the method is and does.
- A syntax section with a pseudo code block with an explanation of it.
- An example showing the method in use.

Please refer to the concept entry templates, content standards, and markdown style guide when working on the PR for this issue.

Entry Type (select all that apply)

New Term for Existing Concept Entry

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[Nithin]

Hi! I'd like to work on this. Please assign it to me

[Daksha Deep]

@nithin-sudarsan You're assigned!

[github-actions]

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