text: Title: Support `separator=<value>` configuration option with scalar variables in Variables section
  Body: In the Variables section, if a scalar variable is created so that it gets more then one value, values are concatenated together so that the final value will be a single sting. By default values are joined with a space, but using a special `SEPARATOR=<value>` marker as the first value makes it possible to change it.
  ```robotframework
  *** Variables ***
  ${JOINED}       These    values    are    joined    together    with    a    space.
  ${MULTILINE}    SEPARATOR=\n    First line.    Second line.    Third line.
  ```
  We are going to introduce new `VAR` syntax (#3761) for creating variables inside tests and keywords, and plan to support concatenation of scalar values with it as well. That syntax gets a separate `scope` option for controlling the scope where the variable is set, and also the separator will be set using a [separator` option](https://github.com/robotframework/robotframework/issues/3761#issuecomment-1757525898) instead of using a special marker like above.
  
  For consistency reasons, we should support the `separator` option also in the Variables section. In practice it would mean that the latter example above could be written like this:
  ```robotframework
  *** Variables ***
  ${MULTILINE}    First line.    Second line.    Third line.    separator=\n
  ```
  Having two ways to control the separator isn't optimal, but there being a consistent syntax that can be used regardless how variables are created is in my opinion more important. We may consider deprecating the `SEPARATOR` marker at some point, but there's absolutely no hurry with it.