Add exponential function spacing option

[pullfrog]

Summary

This PR adds a new --exponential-function-spacing flag that applies exponentially increasing blank lines between function definitions to discourage developers from creating too many functions in a single file.

Behavior

When enabled, the spacing follows the pattern 2^n where n is the function index (0-based):

• 1st to 2nd function: 1 blank line (2^0)
• 2nd to 3rd function: 2 blank lines (2^1)
• 3rd to 4th function: 4 blank lines (2^2)
• 4th to 5th function: 8 blank lines (2^3)
• And so on...

Example

Before:

def first_function():
    pass

def second_function():
    pass

def third_function():
    pass

After (with --exponential-function-spacing):

def first_function():
    pass

def second_function():
    pass


def third_function():
    pass

Changes

• Added exponential_function_spacing boolean field to Mode class
• Updated Mode.__hash__ and get_cache_key to include new field
• Added function_count_at_depth tracking to EmptyLineTracker
• Modified empty line calculation logic to apply exponential spacing
• Added --exponential-function-spacing CLI option

Test Plan

• All existing tests pass
• Manual testing with sample files confirms exponential spacing behavior
• Feature works correctly at different indentation depths

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[pullfrog]

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[github-actions]

diff-shades reports zero changes comparing this PR (7915bc6) to main (b8c15b1).

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