feat: add common string presets

Add token, hex, and pin helpers for the common ASCII string formats already
represented by the crate's built-in charsets. These methods keep frequent use
cases concise while continuing to route through string_from, so callers still
get the same uniform per-character sampling behavior.

Avoid adding a password preset in this change. Password generation implies
policy choices around symbols, ambiguous characters, and service-specific
constraints, while these presets are direct names for existing alphabets.

Document every preset with doctested examples and add a runnable presets
example. Update the demo to show both presets and custom string generation.

Test Plan:
- cargo test
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt

Refs: IDEAS.md ergonomics backlog

examples/demo.rs

@@ -14,13 +14,16 @@ fn main() -> std::io::Result<()> {
     println!("offset   : {}", rng.gen_range_i32_in(-10..=10)?);
     println!("index    : {}", rng.gen_range_usize_in(0..16)?);
 
-    let token = rng.string_from(charset::ALPHANUMERIC, 24)?;
+    let token = rng.token(24)?;
     println!("token    : {token}");
 
-    let hex = rng.string_from(charset::HEX_LOWER, 32)?;
+    let hex = rng.hex(32)?;
     println!("hex      : {hex}");
 
-    let pin = rng.string_from(charset::DIGITS, 6)?;
+    let custom = rng.string_from(charset::ALPHANUMERIC, 12)?;
+    println!("custom   : {custom}");
+
+    let pin = rng.pin(6)?;
     println!("pin      : {pin}");
 
     Ok(())