text: Title: Add explicit command line option to limit which files are parsed
  Body: When execution just some tests from a big directory structure, parsing only relevant files instead of the whole structure makes parsing faster. We don't currently have any explicit option for limiting parsing like that, but when the `--suite` option is used files not matching the specified suite are ignored. This functionality won't work too well in the future, though, if we make it possible to change the suite name using the `Name` setting (#4583). It's also likely that not too many users know about this functionality and the underlying logic to convert file and directory names to suite names for matching purposes is also rather complicated.
  
  A good way to solve all the above problems is adding a separate command line option or options for controlling what files to parse. Once we have such functionality, we can remove the current performance optimization from `--suite`, a separate option makes the feature easier to discover, and also the code gets simpler. There are some design decisions to be made, though:
  
  - What should the option be named? I was thinking `--includefiles`, but it has the same prefix as `--include` which would mean that shortened usages like `--incl` that used to be unique wouldn't work anymore. Perhaps `--parsefiles` or just `--files` would be better.
  - Should the value be a literal value or should we accept glob patterns? I believe supporting patterns is a good idea.
  - Should the value be just a file name or a relative or absolute path to it? I see benefits with both so possibly we could support both usages.
  - Should the value include file extension? I believe it should be allowed but preferably not required. If that gets too complicated, always requiring it is probably better.
  - Should there be separate option for excluding files? It would make some use cases easier, but I'm not sure is that worth the added complexity.
  - Should this option affect only files or also directories? If the option would match a directory, we could automatically include all files under it, recursively. On the other hand, just matching files would be easier to implement and explain.
  - Should we have separate options for matching files and directories? That's probably too much, especially if we'd have separate options for including and excluding. If directories are considered important, probably better to implement this so that the same option work with both files and directories.
  
  This feature should be implemented before RF 7 where we'd like to change `--suite` so that it doesn't affect parsing. That would give users who need the performance optimization time to update their scripts.
  