Adds constant builder (#103)

## Description

Adds a `.constant(JSONValue)` modifier

```swift
   @JSONSchemaBuilder var sample: some JSONSchemaComponent {
      JSONString()
        .constant("red")
    }
```

## Type of Change

- [ ] Bug fix
- [x] New feature
- [ ] Breaking change
- [ ] Documentation update

## Additional Notes

Add any other context or screenshots about the pull request here.

Author: ajevans99

Sources/JSONSchemaBuilder/JSONComponent/Modifier/Constant.swift

@@ -0,0 +1,9 @@
+import JSONSchema
+
+extension JSONSchemaComponent {
+  public func constant(_ value: JSONValue) -> Self {
+    var copy = self
+    copy.schemaValue[Keywords.Constant.name] = value
+    return copy
+  }
+}

Tests/JSONSchemaBuilderTests/JSONEnumTests.swift renamed to Tests/JSONSchemaBuilderTests/JSONModifierTests.swift

@@ -1,8 +1,7 @@
 import JSONSchema
+import JSONSchemaBuilder
 import Testing
 
-@testable import JSONSchemaBuilder
-
 struct JSONEnumTests {
   @Test func singleValue() {
     @JSONSchemaBuilder var sample: some JSONSchemaComponent {
@@ -75,3 +74,32 @@ struct JSONEnumTests {
     #expect(sample.schemaValue == .object(expected))
   }
 }
+
+struct JSONConstantTests {
+  @Test func constantOnly() {
+    @JSONSchemaBuilder var sample: some JSONSchemaComponent {
+      JSONAnyValue()
+        .constant("red")
+    }
+
+    let expected: [String: JSONValue] = [
+      "const": "red"
+    ]
+
+    #expect(sample.schemaValue == .object(expected))
+  }
+
+  @Test func string() {
+    @JSONSchemaBuilder var sample: some JSONSchemaComponent {
+      JSONString()
+        .constant("red")
+    }
+
+    let expected: [String: JSONValue] = [
+      "type": "string",
+      "const": "red",
+    ]
+
+    #expect(sample.schemaValue == .object(expected))
+  }
+}