fix: parse multiple route params in same part

[marvinhagemeister]

Our existing way to parse file paths to patterns for routes was a bit rudimentary. It only allowed parameters to be the full path segments:

Before:

• foo/[bar]@foo -> foo/[bar]@foo

After:

• foo/[bar]@foo -> foo/:bar@foo

Fixes #1516

[bartlomieju]

LGTM