Fix phpstan issues by making Escher generic

[BackEndTea]

This is:

• refactoring

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Extra checks are currently needed on the result of the Escher::load method, as its not clear what the return type becomes.

By making this class generic, we allow to infer the return type from the constructor argument, for static analysis tools.

Additional get...OrThrow methods have been added, to throw more precise errors, rather than Fail due to a method call on null.

[oleibman]

Thank you for an interesting improvement. Please make it a little better by avoiding new uncovered code. Instead of:

    public function getDggContainerOrThrow(): Escher\DggContainer
    {
        if ($this->dggContainer !== null) {
            return $this->dggContainer;
        }

        throw new SpreadsheetException('dggContainer is unexpectedly null');
    }

Please try something like:

    public function getDggContainerOrThrow(): Escher\DggContainer
    {
            return $this->dggContainer ?? throw new SpreadsheetException('dggContainer is unexpectedly null');
    }