[Term Entry] Dart Type Conversion .toStringAsExponential()

[codecademy-docs]

Reason/inspiration (optional)

We would like a new term entry under Dart for the Dart Type Conversion .toStringAsExponential() method.
Provide the entry in a markdown file under docs/content/dart/concepts/type-conversion/terms/toStringAsExponential/toStringAsExponential.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 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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