[Tech] Use compile-time constants for isWindows, isMac and isLinux

[CommandMC]

When building Heroic, we know which platform we're building for at all times (the current platform when running pnpm start, the target platform when running dist:win, dist:mac, etc.). Because of this, we can tell Vite about that platform (with a special mode), and in turn replace constants like isWindows with literal values (true/false). This then allows Rollup to do tree-shaking (it will completely eliminate code paths and even entire dependencies if they're not used)

Rollup sadly seems rather peculiar about when it'll do this tree-shaking:

if (false)
  console.error('this should not happen!')

gets removed as expected, but

switch (false) {
  case true:
    console.error('this should not happen!')
    break
}

does not. As such, we'll have to move towards if-based checks (if (isWindows) { /* ... */ } else if (isMac) { /* ... */ }) instead of switch-based checks (switch (process.platform) { case 'win32': /* ... */ }) to get the most out of this feature

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)