ENH: _font: Set some defaults for the FontDescriptor class

This patch introduces some defaults for the FontDescriptor. This
might be useful if we would like to migrate existing code for
collecting font widths to the class, while not having all code
in place to also collect all associated other information about
the font.

The defaults I took from Google Gemini...

Author: PJBrs

pypdf/_font.py

@@ -1,23 +1,26 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 
 @dataclass(frozen=True)
 class FontDescriptor:
     """
     Represents the FontDescriptor dictionary as defined in the PDF specification.
     This contains both descriptive and metric information.
+
+    The defaults are derived from the mean values of the 14 core fonts, rounded
+    to 100.
     """
 
-    name: str
-    family: str
-    weight: str
+    name: str = "Unknown"
+    family: str = "Unknown"
+    weight: str = "Unknown"
 
-    ascent: float
-    descent: float
-    cap_height: float
-    x_height: float
-    italic_angle: float
-    flags: int
-    bbox: tuple[float, float, float, float]
+    ascent: float = 700.0
+    descent: float = -200.0
+    cap_height: float = 600.0
+    x_height: float = 500.0
+    italic_angle: float = 0.0  # Non-italic
+    flags: int = 32  # Non-serif, non-symbolic, not fixed width
+    bbox: tuple[float, float, float, float] = field(default_factory=lambda: (-100.0, -200.0, 1000.0, 900.0))
 
-    character_widths: dict[str, int]
+    character_widths: dict[str, int] = field(default_factory=dict)