Detect the 'entry' script in F# scripts

I propose we add syntax to detect whether the current F# script is being run as the entry-point

```F#
#if __MAIN__
.. Main code here
#endif
```

In Python it is quite easy - and very *very* common to include code like this

```python
if __name__ == "__main__":
...
```

The reason it exists is to facilitate composability; being able to reuse functions of a script in other scripts without actually executing the segment underneath "main".

There is no way to do this presently when running an FSI script (correct me if I'm wrong)

## Pros and Cons

The advantages of making this adjustment to F# are that it would allow improved re-use of F# scripts by #loading them into other scripts

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this