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

1 files changed, 30 insertions, 0 deletions

diff --git a/tests/pattern/classes.test.py b/tests/pattern/classes.test.py
index de6bfc0..8a82353 100644
--- a/tests/pattern/classes.test.py
+++ b/tests/pattern/classes.test.py
@@ -3,15 +3,19 @@
 import pytest
 
 from edify import (
+    ALPHANUMERIC,
     ANY_CHAR,
     CARRIAGE_RETURN,
     DIGIT,
+    LETTER,
+    LOWERCASE,
     NEW_LINE,
     NON_DIGIT,
     NON_WHITESPACE,
     NON_WORD,
     NULL_BYTE,
     TAB,
+    UPPERCASE,
     WHITESPACE,
     WORD,
     Pattern,
@@ -32,6 +36,10 @@ from edify import (
         (CARRIAGE_RETURN, "\\r"),
         (TAB, "\\t"),
         (NULL_BYTE, "\\0"),
+        (LETTER, "[a-zA-Z]"),
+        (UPPERCASE, "[A-Z]"),
+        (LOWERCASE, "[a-z]"),
+        (ALPHANUMERIC, "[a-zA-Z0-9]"),
     ],
 )
 def test_character_class_constant_compiles_to_expected_regex(constant, expected):
@@ -52,7 +60,29 @@ def test_character_class_constant_compiles_to_expected_regex(constant, expected)
         CARRIAGE_RETURN,
         TAB,
         NULL_BYTE,
+        LETTER,
+        UPPERCASE,
+        LOWERCASE,
+        ALPHANUMERIC,
     ],
 )
 def test_character_class_constant_is_a_pattern(constant):
     assert isinstance(constant, Pattern)
+
+
[email protected](
+    ("constant", "hit_input", "miss_input"),
+    [
+        (LETTER, "A", "4"),
+        (LETTER, "z", " "),
+        (UPPERCASE, "Q", "q"),
+        (LOWERCASE, "q", "Q"),
+        (ALPHANUMERIC, "4", " "),
+        (ALPHANUMERIC, "A", "!"),
+    ],
+)
+def test_convenience_char_class_constant_matches_expected_characters(
+    constant, hit_input, miss_input
+):
+    assert constant.test(hit_input) is True
+    assert constant.test(miss_input) is False