refactor: improve type definitions for async/if then

PR-URL: #1381
Closes: #1095 

Signed-off-by: Philipp Burckhardt <pburckhardt@outlook.com>
Co-authored-by: Philipp Burckhardt <pburckhardt@outlook.com>
Reviewed-by: Philipp Burckhardt <pburckhardt@outlook.com>

Author: prajwalkulkarni

lib/node_modules/@stdlib/utils/async/if-else/docs/types/index.d.ts

@@ -24,7 +24,7 @@
 * @param error - encountered error or null
 * @param result - `x` or `y`
 */
-type Callback = ( error: Error | null, result: any ) => void;
+type Callback<T> = ( error: Error | null, result: T ) => void;
 
 /**
 * Predicate callback function.
@@ -61,6 +61,15 @@ type PredicateCallback =  PredicateNullary | PredicateUnary | PredicateBinary;
 */
 type Predicate = ( clbk: PredicateCallback ) => void;
 
+/**
+ * Determines the relationship between two types `T` and `U`, and returns:
+ *
+ * -   `T` if `T` is assignable to `U`,
+ * -   `U` if `U` is assignable to `T`,
+ * -    A union of `U | T` as a fallback.
+ */
+type ResultFunction<T, U> = T extends U ? T : U extends T ? U : U | T;
+
 /**
 * If a predicate function returns a truthy value, returns `x`; otherwise, returns `y`.
 *
@@ -87,7 +96,7 @@ type Predicate = ( clbk: PredicateCallback ) => void;
 * }
 * ifelseAsync( predicate, 1.0, -1.0, done );
 */
-declare function ifelseAsync( predicate: Predicate, x: any, y: any, done: Callback ): void;
+declare function ifelseAsync<T, U = T>( predicate: Predicate, x: T, y: U, done: Callback<ResultFunction<T, U>> ): void;
 
 
 // EXPORTS //