[6.0.x] URL path bug fixes

[jrflat]

Explanation: Fixes various URL path API behaviors to match the expected behavior from before the Swift URL implementation:

• Drops trailing slashes from URL.fileSystemPath and URL.path
• Strips trailing slashes in URL(filePath:) if a user supplies .notDirectory
• Does not treat ~ paths as absolute in URL(filePath:), resolves the tilde and prevents a crash during initialization
• Strips the leading slash of Windows drive letter paths on Windows
• Prevents unexpected path extension API behavior when the path is a directory

Scope: Targeted changes to URL path APIs to maintain compatibility
Original PRs: #852, #867, #961, #964, #965
Risk: Low - Targeted scope, restores expected behavior
Testing: Local, swift-ci from swift, swift-foundation, and swift-corelibs-foundation, stable in main
Reviewers: @jmschonfeld @iCharlesHu

[jrflat]

@swift-ci please test

[parkera]

We'll need these for the 6.0.2 branch too.

[jrflat]

Cherry-picks for 6.0.2 here: #983