Add support for optional params

[ghalle]

This PR adds support for optional params to the file system routing.

The syntax chosen is [[param]], the first set of square brackets indicate that it is a parameter and the second set indicate that it is optional.
This syntax was chosen because ? is not a valid character in filenames on Windows.

Usage example:

src/routes/[[lang]]/blog/test.tsx

path	params
/blog/test	{}
/en/blog/test	{ lang: "en" }

[ryansolid]

Thanks. This has been overdue. I was thinking about it just the other day.

[jchatard]

Hi @ryansolid somewhat related to this PR just in case you didn’t see it: solidjs/solid-router#227

Thanks.