Add check for path aliases

[wout]

Purpose

Adds a compile-time error in case a path alias is not prefixed with a /.

Description

I got bitten by this today. Main routes and prefixes already do this check but not the aliases. I also fixed the punctuation on the other checks because they would render like:

 Error: Path must start with a slash. Example: '/"about"'

Now it's:

 Error: Path must start with a slash. Example: "/about"

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test