fix(defaults): set 'fsync' #26034

Problem:
'nofsync' may lose data if the system has a hard shutdown. #9888

Solution:
Change default to 'fsync'. This may be revisited in the future when
'nofsync' can be made safer.

runtime/doc/options.txt

@@ -2796,7 +2796,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	security reasons.
 
 					*'fsync'* *'fs'* *'nofsync'* *'nofs'*
-'fsync' 'fs'		boolean	(default off)
+'fsync' 'fs'		boolean	(default on)
 			global
 	When on, the OS function fsync() will be called after saving a file
 	(|:write|, |writefile()|, …), |swap-file|, |undo-persistence| and |shada-file|.

runtime/doc/vim_diff.txt

@@ -47,7 +47,6 @@ Defaults					            *nvim-defaults*
 - 'encoding' is UTF-8 (cf. 'fileencoding' for file-content encoding)
 - 'fillchars' defaults (in effect) to "vert:│,fold:·,foldsep:│"
 - 'formatoptions' defaults to "tcqj"
-- 'fsync' is disabled
 - 'hidden' is enabled
 - 'history' defaults to 10000 (the maximum)
 - 'hlsearch' is enabled

src/nvim/options.lua

@@ -3307,7 +3307,7 @@ return {
     },
     {
       abbreviation = 'fs',
-      defaults = { if_true = false },
+      defaults = { if_true = true },
       desc = [=[
         When on, the OS function fsync() will be called after saving a file
         (|:write|, |writefile()|, …), |swap-file|, |undo-persistence| and |shada-file|.

test/functional/core/fileio_spec.lua

@@ -51,6 +51,7 @@ describe('fileio', function()
 
   it('fsync() codepaths #8304', function()
     clear({ args={ '-i', 'Xtest_startup_shada',
+                   '--cmd', 'set nofsync',
                    '--cmd', 'set directory=Xtest_startup_swapdir' } })
 
     -- These cases ALWAYS force fsync (regardless of 'fsync' option):