Read project key from `sonar-project.properties`

### Prerequisites

- [X] This improvement has not already been suggested.
- [X] This improvement would be generally useful, not specific to my code or setup.
- [X] This improvement is not related to analysis quality (e.g. new rules, improvements to existing rules), which should be raised on the [SonarDelphi repository](https://github.com/integrated-application-development/sonar-delphi/issues).

### Engine area

Delphi IDE integration

### Improvement description

If no project key is specified in the DelphiLint project options, read the project key from the `sonar-project.properties` file if present.

### Rationale

DelphiLint currently handles the project key used to connect to SonarQube totally separately from the sonar-project.properties file, which is passed directly to SonarDelphi.

This made sense within the architecture of the application, but can be unintuitive (see discussion [here](https://github.com/integrated-application-development/delphilint/issues/53#issuecomment-2211350264)). It also means that for the user there are two sources of truth for a project key.

Because the `sonar-project.properties` file is trivially parseable, this shouldn't be too difficult.