Make TPJEnvironmentVar record read only / invariant.

[delphidabbler]

There's no need for the user to change any fields of TPJEnvironmentVar, so we can make the record invariant by hiding the fields, adding read only properties and providing a constructor:

TPJEnvironmentVar = record
strict private
  var
    fName: string;
    fValue: string;
public
  constructor Create(const AName, AValue: string);
  ///  <summary>Environment variable name.</summary>
  property Name: string read fName;
  ///  <summary>Environment variable value.</summary>
  property Value: string read fValue;
end;

The constructor will have have the obvious simple implementation.

Wherever the records fields are directly set in the code the constructor will need to be called instead.