[Suggestion]: Generalize Scoping Modifier

[vhyrro]

This suggestion stems from @boltlessengineer's idea on discord. The goal of this suggestion is to generalize the : syntax to make more sense in the context of the Norg specification.

With these new changes, the old scoping modifier (:) is substituted just for : (no surrounding spaces).
When a link starts with :, this starts a scope at the path to the current file. Therefore, {:myfile} is a link to myfile.norg in the current directory. Of course, {:myfile:} is still permitted, it's just that the second scoping modifier does nothing.

This makes the entire syntax much more intuitive. For instance, {:myfile:* Heading} now makes logical sense: first, we enter the scope of the current dir, then myfile.norg, then we limit the scope to just * Heading.

The reason the old syntax required surrounding whitespace is to prevent situations like this:

* My Life: The Truth

This is a link to {* My Life: The Truth}.

With this new syntax proposal the : has a higher precedence within a link, therefore to link to such a heading one would need to escape the colon using \:. This is a fair compromise (there is no better alternative without massively altering syntax).