Feature: Add ability to define declare statements

[icanhazstring]

This will add the ability to configure declare() statements
at the top of generated files.

I took the implementation suggestion from @GaryJones in #102 and changes it a bit.
I changed the behavior to be only able to generate valid declare statements at all.
Also I added static factories to generate the possible directives.

Should solve #102

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One problem tho. Since declare is a reserved keyword. The I decided to go with Declare_ as the class representation. But the codesniffer doesn't like this. Any suggestions? Renamed to DeclareStatement as suggested by @webimpress

[michalbundyra]

@icanhazstring I would go with DeclareStatement

[michalbundyra]

@icanhazstring maybe I missed something but with that implementation is not possible to add declare like:

declare(strict_types=1,ticks=1);

but it's valid. It can have even all three declarations - strict_types, ticks and encodning.

[michalbundyra]

@icanhazstring
Ok, nvm. I see, you are doing it on separate statements. It's fine. Maybe even better :)

[icanhazstring]

@webimpress wanted to write this. Yes ;)
The other way around would be also possible, but I guess this is a matter of taste. I would have to change the FileGenerator to only have a single DeclareStatement. Maybe then we could remove the logic inside setDeclares to check if the declare is already given. Maybe some better SoC? Don't know ;)

[michalbundyra]

Thanks, @icanhazstring!