Feature: Create the StringSchema

[nickytonline]

As a user of ValiSwift, I want a String schema for Valibot in Swift so that I can validate strings.

[nickytonline]

For the technical implementation see the following files in Valibot:

• https://github.com/fabian-hiller/valibot/blob/82c21e205c1e178882fb0517fa39dd2bf7c87357/library/src/schemas/string/string.ts
• https://github.com/fabian-hiller/valibot/blob/0b05a210de9ae04f3086cae2e1bdbf9e38ac55e4/library/src/schemas/string/string.test.ts
• Keep the same folder structure, so this should live in Sources/Schemas in ValiSwift and be sure to add the Schemas folder to the Package.swift as well as the Types folder which is where some types the string schema import from in TypeScript.

  targets: [
    // Targets are the basic building blocks of a package, defining a module or a test suite.
    // Targets can depend on other targets in this package and products from dependencies.
    .target(
-      name: "ValiSwift"),
+      name: "ValiSwift",
+      path: "Sources",
+      sources: ["Schemas", "Types"]),
  ...