Use user-specific cache directory by default

[MukundaKatta]

Summary

• move the implicit encoding download cache from the shared temp directory to a user-specific cache directory ($XDG_CACHE_HOME/tiktoken, %LOCALAPPDATA%/tiktoken, or the platform fallback)
• create the implicit cache directory with 0700 permissions so other local users cannot pre-populate or read it
• add a regression test that verifies the user cache path is used, cached reads avoid refetching, and POSIX permissions are private

Fixes #500.

Tests

• /tmp/tiktoken-secure-cache-venv/bin/python -m pytest tests/test_misc.py -q
• /tmp/tiktoken-secure-cache-venv/bin/python -m pytest tests/test_simple_public.py -q
• /tmp/tiktoken-secure-cache-venv/bin/python -m pytest tests/test_encoding.py -q -k 'not hyp_roundtrip'\n\nNote: the full tests/test_encoding.py suite currently fails independently in test_hyp_roundtrip[cl100k_base] when Hypothesis generates the literal disallowed special token <|fim_prefix|>; the narrower command above excludes that pre-existing property-test case.