Add constructor to `Substring` entity that accepts only `length`

[kudima03]

The Substring entity currently requires two parameters in its constructor: start and length.
It would be useful to add an additional constructor that accepts only the length parameter, while the start parameter defaults to 0.

Proposed change:

• Add a new constructor to Substring with the following signature:

  Substring(INumber<ushort> length)

• Internally, this constructor should call the main constructor with start = 0 and the provided length.

Benefits:

• Simplifies creation of substrings when the starting index is always 0.
• Improves code readability and reduces boilerplate.