Replace Rake's Win32-specific logic with a 100% equivalent, pure-Ruby implementation

[pvdb]

tl;dr - replace some Win32-specific Rake logic for Windows home directory detection logic with Ruby's built-in Dir.home method

Rake's custom Windows-specific implementation (Rake::Win32::win32_system_dir) produces identical results to Ruby's standard library approach (File.join(Dir.home, "Rake")) for all possible scenarios it caters for, so this PR removes a whole bunch of superfluous code and corresponding tests.

It is replaced with a much simpler, pure stdlib-based implementation. in a low-risk manner: Ruby's cross-platform Dir.home has been available since Ruby 1.9 and is the standard way to get the home directory; this change simply stops reinventing the wheel. 😃

1. Simpler code — Removes ~85 lines of custom Win32 path handling code, tests, and a custom exception class
2. More correct — Dir.home is Ruby's official, well-tested cross-platform solution that handles edge cases properly. The old implementation had a hand-rolled fallback chain (HOME → HOMEDRIVE+HOMEPATH → APPDATA → USERPROFILE) that Ruby already handles correctly internally
3. Less maintenance burden — No need to maintain Windows-specific path normalization or environment variable logic that duplicates Ruby stdlib functionality
4. Consistent behavior — Aligns Rake's behavior with how other Ruby tools determine the home directory on Windows

Because it's hard to use and test something that isn't there anymore 😄 I've raised an accompanying DRAFT pull request #670 that explicitly tests the equivalency of both versions against the master branch (for a variety of different scenarios) thereby demonstrating that the Rake-specific patch is superfluous and can be safely removed.

[pvdb]

It is worth noting that the Win32HomeError exception is never thrown IRL, because Ruby will always set the HOME environment variable in its environment, even if it isn't set in the Windows environment the Ruby process is running in.

It has done so since Ruby 1.9.x in 2004: ruby/ruby@c41cefd492

[pvdb]

As per the above comment, this test is superfluous, as the exception will never be thrown IRL.