add .iml file to fix busted intellij module

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Transferred from: mariposa-dart#18

because the .idea/modules.xml file is committed, the .iml file it refers to needs to alos be committed, otherwise the project folders get hidden:

more info: https://youtrack.jetbrains.com/issue/IDEA-281089/directories-not-visible-in-project-view-when-iml-file-cant-be-found

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this was a pr but there was no linked issue

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964f7dc