implement String(::Base.Generator) constructor

[xingzihai]

Summary

This PR implements a String(::Base.Generator) constructor, resolving the method error when attempting to call String on a Generator (e.g., from Iterators.map).

Problem

Currently, calling String(Iterators.map(c->c+1, "Hello, world")) results in a method error because there is no String method that accepts a Generator argument.

Solution

Add a new constructor:

String(g::Generator) = join(g)

This uses join to convert the Generator elements to a String, where each element is converted via print.

Example Usage

# Before: MethodError
String(x for x in ["a", "b", "c"])  # Now works: "abc"

String(Iterators.map(identity, "hello"))  # Now works: "hello"

Notes

• The implementation uses join(g) which is a well-established method for converting iterables to strings.
• Each element is converted to string via print, providing flexibility for different element types.
• A docstring with examples has been added.

Closes #57072