refactor: use tuple for return type

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---

Author: anandkaranubc

lib/node_modules/@stdlib/math/base/special/modff/docs/types/index.d.ts

@@ -36,7 +36,7 @@ interface Modff {
 	* var parts = modff( 3.14 );
 	* // returns [ 3.0, 0.1400001049041748 ]
 	*/
-	( x: number ): Array<number>;
+	( x: number ): [ number, number ];
 
 	/**
 	* Decomposes a single-precision floating-point number into integral and fractional parts, each having the same type and sign as the input value.

lib/node_modules/@stdlib/math/base/special/modff/docs/types/test.ts

@@ -25,7 +25,7 @@ import modff = require( './index' );
 
 // The function returns a numeric array...
 {
-	modff( 1.0 ); // $ExpectType number[]
+	modff( 1.0 ); // $ExpectType [number, number]
 }
 
 // The compiler throws an error if the function is provided an argument other than a number...