ftd.app-path function

[siddhantk232]

This function let's ftd files inside a mounted package construct urls
that are relative to the mountpoint of the said package.

This helps fastn.app mounts to easily construct paths without assuming
(or enforcing) their mountpoints.

Calling ftd.app-path(path = /test/) in an ftd file of a mounted app will return the path
prefixed with the mountpoint of the app.

The path arg must start with a forward slash (/).

Calling this from any .ftd file of the root package will simply return the provided path argument.

Example

-- import: fastn
                                                                                            
-- fastn.package: test
                                                                                            
-- fastn.app: Test
mountpoint: /app/
package: some-test-app.fifthtry.site

                                                                                            
-- ftd.text: $ftd.app-path(path = /test/)

Visiting /app/ in browser should render /app/test/

[amitu]

@siddhantk232 also do the version number bump as we want to create a release right after merging this PR.