Add figure.titlehorizontalalign and figure.titleverticalalign rcParams

Fixes #24090

Problem

suptitle() defaults for horizontal and vertical alignment were hardcoded (center / top). Only fontsize and fontweight were configurable via rcParams. Users who wanted to control suptitle alignment had to call suptitle(ha=..., va=...) every time — no way to set a project-wide default via rc settings.

Solution

Added two new rcParams:

• figure.titlehorizontalalign (default: "center") — validates against {center, left, right, center_left, center_right}
• figure.titleverticalalign (default: "top") — validates against standard vertical alignments

import matplotlib.pyplot as plt

plt.rcParams[figure.titlehorizontalalign] = left
plt.rcParams[figure.titleverticalalign] = bottom

fig, ax = plt.subplots()
fig.suptitle("Left-aligned title")  # Uses rcParams defaults
fig.suptitle("Override", ha=right)  # Explicit kwarg still wins

Changes

• rcsetup.py: Added validate_suptitle_ha validator + two new rcParam entries with defaults
• figure.py: Updated _suplabels() to resolve ha/va from rcParams when the key is set
• matplotlibrc: Added commented-out default values for the new params
• tests/test_figure.py: Two new tests — rcParams alignment applies, kwarg overrides work
• typing.py: Added new keys to RcKeyType enum

Backwards Compatibility

Fully backwards compatible. Default values match the old hardcoded defaults (center / top). Existing code that passes explicit ha/va kwargs is unaffected.