fix: improve dev stop Ctrl+C output with concise shutdown messages @W-21322039@

[deepu-mungamuri94]

@W-21322039@

• Add user-visible shutdown messages (stopping proxy, dev server)
• Use checkmark style (✅ Stopped dev & proxy servers.)
• Clear force-kill timeout in DevServerManager to prevent process hang
• Fix explicit return type for handleSignal (lint)

What does this PR do?

CLI output says to use CTRL+C to stop the dev server, but when you do, you get a RED error like so:
^C/Users/tconn/.local/share/sf/node_modules/@salesforce/plugin-app-dev/node_modules/@oclif/core/lib/errors/exit.js:6
throw new exit_1.ExitError(code);

What issues does this PR fix or reference?

improve dev stop Ctrl+C output with concise shutdown messages instead of error kind of output.

[deepu-mungamuri94]

Previously without fix :

^C/Users/tconn/.local/share/sf/node_modules/@salesforce/plugin-app-dev/node_modules/@oclif/core/lib/errors/exit.js:6
  throw new exit_1.ExitError(code);
  ^
ExitError: EEXIT: 130

  at Object.exit (/Users/tconn/.local/share/sf/node_modules/@salesforce/plugin-app-dev/node_modules/@oclif/core/lib/errors/exit.js:6:11)
  at WebappDev.exit (/Users/tconn/.local/share/sf/node_modules/@salesforce/plugin-app-dev/node_modules/@oclif/core/lib/command.js:219:16)
  at process.<anonymous> (file:///Users/tconn/.local/share/sf/node_modules/@salesforce/plugin-app-dev/node_modules/@salesforce/sf-plugins-core/lib/sfCommand.js:274:22)
  at process.emit (node:events:520:35) {
 code: 'EEXIT',
 oclif: { exit: 130 },
 skipOclifErrorHandling: undefined,
 suggestions: undefined
}

After fix: we have clean exit.