Add active pattern for empty computation expression

[nojaf]

Fixes #3140

I took a bit of a shortcut, that is why there is an additional space after formatting.
This might be ok to for now.

[nojaf]

The generated lex.fsi no longer seems valid.
module Ranges =/// Rule token is invalid according to the latest SDK.
I'm not sure why this generating to no longer working.

@Martin521 does this ring a bell? Did anything change in that area?

[Martin521]

No, I am not aware of changes.
What exactly do you mean by your comment? Does compilation of the generated lex.fs create errors? And the error message is module Ranges =/// Rule token?
module Ranges and //Rule token are in quite different places in lex.fs.

[nojaf]

Yes, the generated lex.fs contains invalid code and compilation fails.
I can't quite piece together why this is happening now.
Maybe you have seen this before, since you did some work in that area.