feat: convenience char classes .letter() .uppercase() .lowercase() .alphanumeric() (#267)

Adds four convenience ASCII character-class shortcuts so callers don't
have to spell out `.range("a","z").range("A","Z")` for the common cases.

## What is new
- **Chain methods** on `ClassesMixin` — `.letter()` → `[a-zA-Z]`,
`.uppercase()` → `[A-Z]`, `.lowercase()` → `[a-z]`, `.alphanumeric()` →
`[a-zA-Z0-9]`.
- **Leaf elements** — `LetterElement`, `UppercaseElement`,
`LowercaseElement`, `AlphanumericElement` in
`edify.elements.types.leaves`, joined into `LeafElement` and `Element`
unions and rendered by `edify.compile.leaves.render_leaf`.
- **Module constants** — `LETTER`, `UPPERCASE`, `LOWERCASE`,
`ALPHANUMERIC` at both `edify.pattern` and the top-level `edify`
namespace.

## Example
```python
from edify import LETTER, ALPHANUMERIC, START, END, one_or_more

username = START + one_or_more(ALPHANUMERIC) + END
LETTER.test("Q")   # True
LETTER.test("4")   # False
```

Follows the same dedicated-leaf pattern the existing `.digit()` /
`.word()` / `.whitespace_char()` methods use.

Closes #132

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