Condition adoption of writing system on modifier

[moretrim]

The purpose of the establish_writing_system decision (typo in the name
fixed in this change) is to remove the crippling lacks_writing_system
modifier when applicable. However the decision is only conditioned on
the related lacking_writing_system flag and not the modifier itself.

Should a country lose the flag in some fashion, it would be stuck with
the debilitating modifier for the rest of the game. I don't believe that
this status can realistically be achieved in most games, though it is
not entirely outside the realm of possibility either. For instance an
organised ARO (aka Biafra) that managed impossibly high literacy
through colonial shenanigans might become eligible for colonial_legacy
after release.

With this change the condition for the establish_writing_system
decision correctly reflects its effect, namely by targeting the
lacks_writing_system modifier. On top of that the modifier is also
consistently removed whenever the flag is.

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This PR has been tested in artificial conditions, in countries with and without a writing system. (Events and Westernisation through RP points via console.)

[moretrim]

For full disclosure I ran into this when I was working on a submod that Westernises every uncivilised country at the start of the game. I tried to make the submod as agnostic as possible to support as many mods as possible, but I ran into this.

As a consequence this PR is more for my own benefit than the mod's, though I still think it improves the codebase overall.

[arkhometha]

Hi @moretrim, thanks for the commit.

Even if not a big deal, it eases your work and apparently can potentially detect the situations you described. One thing though, perhaps would be better to make it so checks either the flag or the modifier for the potential part? This way if a country is left stuck with the flag (and not the modifier) for some reason they can also get rid of it. But that's a change I can make myself.