Add stream construct

[grainier]

Purpose

$title

Fixes #20745

Approach

n/a

Samples

int i = 0;

stream<int> evenNumberStream = stream {
    i += 2;
    return { value: i };
};

var evenNumberRec = evenNumberStream.next();
int evenNumber = <int>evenNumberRec["value"];

Remarks

#20745
ballerina-platform/ballerina-spec#406

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples