Type import for oneOf models in ts-fetch template

[Xowap]

Required for some compilation modes of TS, specifically for use with SvelteKit.

The error I'm getting when using the generated code on SvelteKit is:

TS1444:  XXX  is a type and must be imported using a type-only import when  preserveValueImports  and  isolatedModules  are both enabled.

Those options are deliberate and mandatory according to SvelteKit's doc.

As such I've made a change to the way types are imported, which actually already seems to be present in modelGeneric. This change is conclusive on my end and makes the generated code work in my project.

Now, to be perfectly transparent, I am extremely not used to the tooling of this project and I have no idea if did everything right. Specifically, I think I managed to run some tests but it stays pretty elusive to me, so let me know what/how I should fix it (and how can I avoid having cargo and a million other tools installed on my machine).

This seems to be a PR for @CodeNinjai, @frol and @cliffano

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[wing328]

cc @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04)