feat: standard schema for validation (#42)

* feat: standard schema for validation

* Apply suggestions from code review

Signed-off-by: Florian Lefebvre <contact@florian-lefebvre.dev>

---------

Signed-off-by: Florian Lefebvre <contact@florian-lefebvre.dev>
Co-authored-by: paul valladares <85648028+dreyfus92@users.noreply.github.com>

Author: florian-lefebvre

src/content/docs/clack/guides/best-practices.mdx

@@ -53,6 +53,18 @@ const age = await text({
 });
 ```
 
+A [Standard Schema](https://github.com/standard-schema/standard-schema) can be used, for example using [Arktype](https://arktype.io/):
+
+```ts twoslash
+import { text } from '@clack/prompts';
+import { type } from 'arktype';
+
+const name = await text({
+	message: 'Enter your name (letters only)',
+  validate: type('string.alpha').describe('Name can only contain letters'),
+});
+```
+
 ### 3. Consistent Messaging
 
 Use clear and consistent messaging across your prompts:

src/content/docs/clack/packages/prompts.mdx

@@ -130,7 +130,7 @@ Options:
 - `placeholder`: A visual hint shown when the field has no content.
 - `defaultValue`: A fallback value returned when the user provides nothing (empty input).
 - `initialValue`: The starting value shown when the prompt first renders. Users can edit this value before submitting.
-- `validate`: A function that validates user input. Return a `string` or `Error` to show as a validation error, or `undefined` to accept the result.
+- `validate`: A function or a [Standard Schema](https://github.com/standard-schema/standard-schema) that validates user input. If a custom function is given, you should return a `string` or `Error` to show as a validation error, or `undefined` to accept the result..
 - All [Common Options](#common-options)
 
 ### Password Input
@@ -163,7 +163,7 @@ Options:
 
 - `message`: The prompt message or question shown to the user above the input.
 - `mask`: Character to use for masking input. Default: `'▪/•'`.
-- `validate`: A function that validates user input. Return a `string` or `Error` to show as a validation error, or `undefined` to accept the result.
+- `validate`: A function or a [Standard Schema](https://github.com/standard-schema/standard-schema) that validates user input. If a custom function is given, you should return a `string` or `Error` to show as a validation error, or `undefined` to accept the result.
 - `clearOnError`: When enabled it causes the input to be cleared if/when validation fails. Default: `false`.
 - All [Common Options](#common-options)