mergetools: vscode: new tool

mergetools: vscode: new tool

VSCode has supported three-way merges since 2022, see
<https://github.com/microsoft/vscode/issues/5770#issuecomment-1188658476>.

Although the program binary is located at /usr/bin/code, name the
mergetool "vscode" because the word "code" is too generic and would lead
to confusion. The name "vscode" also matches Git's existing
contrib/vscode directory.

On Windows, VSCode adds the directory that contains code.cmd to %PATH%,
so there is no need to invoke mergetool_find_win32_cmd to search for the
program.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>