os: add os.path_devnull (containing /dev/null on POSIX and \\.\nul on Windows) (#20439)

vlib/os/file_test.v

@@ -477,3 +477,18 @@ fn test_open_file_crlf_binary_mode() {
 
 	assert fcont_wb == teststr
 }
+
+fn test_path_devnull() {
+	dump(os.path_devnull)
+	content := os.read_file(os.path_devnull)!
+	// dump(content)
+	// dump(content.len)
+
+	os.write_file(os.path_devnull, 'something')!
+
+	content_after := os.read_file(os.path_devnull)!
+	// dump(content_after)
+	// dump(content_after.len)
+	assert content.len == 0
+	assert content_after.len == 0
+}

vlib/os/os.js.v

@@ -8,6 +8,8 @@ $if js_node {
 
 pub const path_delimiter = get_path_delimiter()
 pub const path_separator = get_path_separator()
+pub const path_devnull = '/dev/null' // TODO
+
 pub const args = []string{}
 
 const executable_suffixes = ['']

vlib/os/os_nix.c.v

@@ -9,9 +9,18 @@ import strings
 #include <sys/types.h>
 #include <utime.h>
 
+// path_separator is the platform specific separator string, used between the folders
+// and filenames in a path. It is '/' on POSIX, and '\\' on Windows.
 pub const path_separator = '/'
+
+// path_delimiter is the platform specific delimiter string, used between the paths
+// in environment variables like PATH. It is ':' on POSIX, and ';' on Windows.
 pub const path_delimiter = ':'
 
+// path_devnull is a platform-specific file path of the null device.
+// It is '/dev/null' on POSIX, and r'\\.\nul' on Windows.
+pub const path_devnull = '/dev/null'
+
 const executable_suffixes = ['']
 
 const stdin_value = 0

vlib/os/os_windows.c.v

@@ -6,6 +6,18 @@ import strings
 #include <process.h>
 #include <sys/utime.h>
 
+// path_separator is the platform specific separator string, used between the folders
+// and filenames in a path. It is '/' on POSIX, and '\\' on Windows.
+pub const path_separator = '\\'
+
+// path_delimiter is the platform specific delimiter string, used between the paths
+// in environment variables like PATH. It is ':' on POSIX, and ';' on Windows.
+pub const path_delimiter = ';'
+
+// path_devnull is a platform-specific file path of the null device.
+// It is '/dev/null' on POSIX, and r'\\.\nul' on Windows.
+pub const path_devnull = r'\\.\nul'
+
 // See https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createsymboliclinkw
 fn C.CreateSymbolicLinkW(&u16, &u16, u32) int
 
@@ -16,9 +28,6 @@ fn C._getpid() int
 
 const executable_suffixes = ['.exe', '.bat', '.cmd', '']
 
-pub const path_separator = '\\'
-pub const path_delimiter = ';'
-
 // Ref - https://docs.microsoft.com/en-us/windows/desktop/winprog/windows-data-types
 // A handle to an object.
 pub type HANDLE = voidptr