refactor: Windows tilde expansion followup (#29380)

Followup to #28515:

Rename the static os_homedir() to os_uv_homedir() to emphasize that it
is a wrapper around a libuv function.

Add the function os_get_homedir() to os/env.c to return the cached
homedir value as a const. Must be called after homedir is initialized or
it fails.

The difference between this function and the static os_uv_homedir() is
that the latter gets the homedir from libuv and is used to initialize
homedir in init_homedir(), while os_get_homedir() just returns homedir
as a const if it's initialized and is public.

Use the os_get_homedir() accessor for ~/ expansion on Windows to make
the code more concise.

Add a Windows section to main_spec.lua with tests for expanding ~/ and
~\ prefixes for files passed in on the command-line.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

test/functional/core/main_spec.lua

@@ -193,4 +193,26 @@ describe('command-line option', function()
     matches('Run "nvim %-V1 %-v"', fn.system({ nvim_prog_abs(), '-v' }))
     matches('Compilation: .*Run :checkhealth', fn.system({ nvim_prog_abs(), '-V1', '-v' }))
   end)
+
+  if is_os('win') then
+    for _, prefix in ipairs({ '~/', '~\\' }) do
+      it('expands ' .. prefix .. ' on Windows', function()
+        local fname = os.getenv('USERPROFILE') .. '\\nvim_test.txt'
+        finally(function()
+          os.remove(fname)
+        end)
+        write_file(fname, 'some text')
+        eq(
+          'some text',
+          fn.system({
+            nvim_prog_abs(),
+            '-es',
+            '+%print',
+            '+q',
+            prefix .. 'nvim_test.txt',
+          }):gsub('\n', '')
+        )
+      end)
+    end
+  end
 end)