Replace getDefaultShell and mergeDefaultShellPathAndArgs with getDefaultShellAndArgs

[Tyriar]

All consumers of these calls wanted both shell and args, so instead of having both, move to a more simpler approach that just requested the defaults. There are 3 ways that shell and args can now be fetched:

• Local: electron-browser/ITerminalInstanceService calls directly (we don't want to move this to ext host as that would delay the first terminal creation until after the ext host is up)
• Web: browser/ITerminalInstanceService proxies the call to the ext host
• Remote: ExtHostTerminalService calls directly when the process is being created on the ext host

This also renames getDefaultShell in node/terminal.ts to getSystemShell in order to differentiate the system shell (detected system default) with the default shell (default for vscode, based on settings + system).

Fixes #75795