Fix importOrderCombineTypeAndValueImports for Flow

[IanVS]

Fixes https://github.com/IanVS/prettier-plugin-sort-imports/issues/56

I needed to loosen the assertion a bit, because Flow's AST does not apparently use an importKind of "value" for specifiers, instead it is null.

I also added an e2e test here, for both Flow and TypeScript.

[IanVS]

@andidev, if you'd like to test this out in your project, you can find this package in your node_modules and update convertImportSpecifierToType in lib/src/utils/merge-nodes-with-matching-flavors.js to:

function convertImportSpecifierToType(node) {
    (0, assert_1.default)(node.importKind === 'value' ||
        node.importKind === 'type' ||
        // importKind can be null when using Flow
        node.importKind === null);
    node.importKind = 'type';
}

[IanVS]

Thanks @fbartho. I'll give @andidev, a bit longer to hopefully test this out, because I have a feeling there could be other issues with Flow, and it would be good to confirm whether that's the case or not.

[andidev]

We have a large code base and it seems to work well except this setting. But maybe there is more specific cases related to this setting. But to be honest ai don't think so.