🏷️ Better typings for constant (#4999)

**Description**

As recommended by #4047,
let's leverage const generics to make usage of `constant` even simpler
for TS users.

Not really a breaking change on its own but given we play with types and
change the way we declare some, let's be safe and pack this change with
other "next major" devs.

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- [x] Category: 🏷️ Add or update types
- [x] Impacts: Typings

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packages/fast-check/src/arbitrary/constant.ts

@@ -7,6 +7,6 @@ import { ConstantArbitrary } from './_internals/ConstantArbitrary';
  * @remarks Since 0.0.1
  * @public
  */
-export function constant<T>(value: T): Arbitrary<T> {
+export function constant<const T>(value: T): Arbitrary<T> {
   return new ConstantArbitrary([value]);
 }

packages/fast-check/test/e2e/RecursiveStructures.spec.ts

@@ -40,7 +40,7 @@ describe(`RecursiveStructures (seed: ${seed})`, () => {
   it('Should shrink memo/oneof towards the smallest case (on very simple scenario)', () => {
     // Arrange
     const failingLength = 2;
-    const dataArb: fc.Memo<unknown[]> = fc.memo((n) => {
+    const dataArb: fc.Memo<readonly unknown[]> = fc.memo((n) => {
       if (n <= 1) return fc.constant([0]);
       else return fc.oneof({ withCrossShrink: true }, fc.constant([0]), fc.tuple(dataArb(), dataArb()));
     });
@@ -125,7 +125,7 @@ describe(`RecursiveStructures (seed: ${seed})`, () => {
 
 // Helpers
 
-function flat(arr: unknown[]): unknown[] {
+function flat(arr: readonly unknown[]): unknown[] {
   return arr.reduce((acc: unknown[], cur: unknown) => {
     if (Array.isArray(cur)) acc.push(...flat(cur));
     else acc.push(cur);

packages/fast-check/test/unit/arbitrary/webUrl.spec.ts

@@ -119,7 +119,7 @@ describe('webUrl (integration)', () => {
 function webUrlConstraintsBuilder(onlySmall?: boolean): fc.Arbitrary<WebUrlConstraints> {
   return fc.record(
     {
-      validSchemes: fc.constant(['ftp']),
+      validSchemes: fc.constant<string[]>(['ftp']),
       authoritySettings: fc.record(
         {
           withIPv4: fc.boolean(),