Use const enums in TS generator

[DeTeam]

Currently graphql enums are modelled in TS using normal enums. In order to not have any runtime consequences we can switch to const enum (https://www.typescriptlang.org/docs/handbook/enums.html#const-enums)

• feat(docs): updates install docs rover#247
• docs/infra: setup apollographql.com/docs/rover link rover#193

[LinusU]

(crossposting from #1827)

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Note that issues can arise when using const enums if care is not taken, even the TypeScript PM suggests that:

Sure, they can be great for the internals of a library to get some speed improvements, but they generally shouldn't be used outside of that scope.

ref: facebook/create-react-app#4837 (comment)

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The TypeScript team development lead has also chimed in and said that using string unions is the best approach for this project:

From a TypeScript language perspective, this is a big improvement. 👍

ref: #1750 (review)

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Also, const enums doesn't work with Babel which many people are using to transpile their TypeScript code...

ref: https://babeljs.io/docs/en/babel-plugin-transform-typescript#caveats