fix: declare types in .ts files (#3840)

`.d.ts` files are not type checked as they are usually machine generated.

Switch to using `.ts` files instead so we get some compiler safety.

Refs: ipfs/aegir#849

src/types.d.ts → src/types.ts

@@ -1,6 +1,6 @@
-import { Agent as HttpAgent } from 'http'
-import { Agent as HttpsAgent } from 'https'
-import { Multiaddr } from 'multiaddr'
+import type { Agent as HttpAgent } from 'http'
+import type { Agent as HttpsAgent } from 'https'
+import type { Multiaddr } from 'multiaddr'
 import type { BlockCodec } from 'multiformats/codecs/interface'
 import type { MultihashHasher } from 'multiformats/hashes/interface'
 import type { MultibaseCodec } from 'multiformats/bases/interface'