Better Dependabot support in composer manifest

[sgerrand]

💁 These changes were inspired by @timrogers' efforts in duffelhq/duffel-api-javascript#934 to help Dependabot understand the relationships between the project and its dependencies.

I don't know whether this library is still maintained but if it is then this change will help. As an example, these dependencies are all out of date and Dependabot should pick that up after this changeset is merged:

$ composer outdated
Legend:
! patch or minor release available - update recommended
~ major release available - update possible

Direct dependencies required in composer.json:
ergebnis/phpstan-rules             1.0.0  ~ 2.2.0  Provides additional rules for phpstan/phpstan.
guzzlehttp/psr7                    1.9.1  ~ 2.6.2  PSR-7 message implementation that also provides common utility methods
phpunit/phpunit                    9.6.19 ~ 11.2.6 The PHP Unit Testing framework.
psr/cache                          2.0.0  ~ 3.0.0  Common interface for caching libraries
psr/http-message                   1.1    ~ 2.0    Common interface for HTTP messages
symfony/options-resolver           6.4.8  ~ 7.1.1  Provides an improved replacement for the array_replace PHP function
vimeo/psalm                        4.30.0 ~ 5.25.0 A static analysis tool for finding errors in PHP applications

Relevant documentation and code:

• relevant part of the composer.json schema
• Dependabot implementation for composer

[sgerrand]

FYI @igorp1 👋

[sgerrand]

😿 I think that the DUFFEL_ACCESS_TOKEN has disappeared from the secrets in this repository. It'll need to be configured again for the Examples / Code Examples job in the workflow to run successfully.

[timrogers]

LGTM! 🚢