[@koa/router] Add definitions

[jdforsythe]

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This is to add types for @koa/router, recently forked by koajs at @koa/router and available on npm as @koa/router

Fixes #36465

[typescript-bot]

@jdforsythe Thank you for submitting this PR!

Because this is a new definition, a DefinitelyTyped maintainer will be reviewing this PR in the next few days once the Travis CI build passes.

In the meantime, if the build fails or a merge conflict occurs, I'll let you know. Have a nice day!

[typescript-bot]

👋 Hi there! I’ve run some quick performance metrics against master and your PR. This is still an experiment, so don’t panic if I say something crazy! I’m still learning how to interpret these metrics.

Let’s review the numbers, shall we?

These typings are for a package that doesn’t yet exist on master, so I don’t have anything to compare against yet! In the future, I’ll be able to compare PRs to koa__router with its source on master.

Comparison details 📊

Batch compilation
Type count	10079
Assignability cache size	997
Subtype cache size	93
Identity cache size	8
Language service measurements
Samples taken	395
Identifiers in tests	395
getCompletionsAtPosition
Mean duration (ms)	381.5
Median duration (ms)	376.5
Mean CV	11.5%
Worst duration (ms)	488.7
Worst identifier	state
getQuickInfoAtPosition
Mean duration (ms)	413.9
Median duration (ms)	409.2
Mean CV	14.0%
Worst duration (ms)	542.1
Worst identifier	routeHandler4
System information
Node version	v10.16.0
CPU count	2
CPU speed	2.294 GHz
CPU model	Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz
CPU Architecture	x64
Memory	6.8 GiB
Platform	linux
Release	4.15.0-1047-azure

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If you have any questions or comments about me, you can ping @andrewbranch. Have a nice day!

[andrewbranch]

👍 works for me

[typescript-bot]

I just published @types/koa__router@8.0.0 to npm.