fix: assert type in decodeJWTPayload (#1018)

## What kind of change does this PR introduce?

Bug fix

## What is the current behavior?

Type checking and/or linting does not pass for some configurations, per
#967 and
#1017

## What is the new behavior?

We're now asserting that `parts[1]` is a string (aka isn't undefined),
since we know from a previous check that the array length is 3.

## Additional context

Fixes #967 and fixes #1017

Co-authored-by: Kang Ming <[email protected]>

Author: j4w8n

Diff for: src/lib/helpers.ts

@@ -205,11 +205,11 @@ export function decodeJWTPayload(token: string) {
     throw new Error('JWT is not valid: not a JWT structure')
   }
 
-  if (!base64UrlRegex.test(parts[1])) {
+  if (!base64UrlRegex.test(parts[1] as string)) {
     throw new Error('JWT is not valid: payload is not in base64url format')
   }
 
-  const base64Url = parts[1]
+  const base64Url = parts[1] as string
   return JSON.parse(decodeBase64URL(base64Url))
 }