Update TypeScript.md

[mikicho]

Fix #4122

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

I think we need to elaborate more on this area.

TypeScript can only identify the types that are direct imported in code which means that you can use require inline with import on top.

For example,

import 'plugin' // here will trigger the type augmentation.

fastify.register(require('plugin'))

import plugin from 'plugin' //  here will trigger the type augmentation.
fastify.register(plugin)

Or even explicit config on tsconfig

{
  "types": ["plugin"] // we force TypeScript to import the types
}

[mcollina]

lgtm

[mikicho]

@climba03003 WDYT?

[climba03003]

LGTM.

Except the line need to be formatted and break in length 80.

[mikicho]

Thanks! @Fdawgs

[mcollina]

lgtm

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