Add support for dynamic field values

[giuscris]

This PR adds the possibility to use dynamic field values.
This feature supersedes the import key, used to call static functions.

From now, field keys ending with @ introduce dynamic values, which are lazily computed when accessed. Values are parsed with dot notation, i.e. formwork.site().currentPage().title() corresponds to $formwork->site()->currentPage()->title().

This syntax is more streamlined, so for example this syntax:

role:
    type: select
    label: '{{panel.user.role}}'
    disabled: true
    import:
        options: 'Formwork\Panel\Users\UserCollection::availableRoles'

Becomes:

role:
    type: select
    label: '{{panel.user.role}}'
    disabled: true
    options@: formwork.panel().users().availableRoles()

In the future the possibility to parse expressions could be added.