[all] Check for VERCEL_ environment variables with getPlatformEnv() (#4274)

Added the following env vars, most are undocumented but its good to be consistent:

- `VERCEL_REGION`
- `VERCEL_DEBUG`
- `VERCEL_BUILDER_DEBUG`
- `VERCEL_TOKEN`
- `__VERCEL_SKIP_DEV_CMD`

I also updated the error code prefixes to remove `NOW_`.
There `code` isn't used anywhere, this is just to make it unique and a little shorter.

packages/now-cli/test/dev/integration.js

@@ -104,7 +104,7 @@ async function exec(directory, args = []) {
   return execa(binaryPath, ['dev', directory, ...args], {
     reject: false,
     shell: true,
-    env: { __NOW_SKIP_DEV_COMMAND: 1 },
+    env: { __VERCEL_SKIP_DEV_CMD: 1 },
   });
 }
 
@@ -180,7 +180,7 @@ async function testFixture(directory, opts = {}, args = []) {
       shell: true,
       stdio: 'pipe',
       ...opts,
-      env: { ...opts.env, __NOW_SKIP_DEV_COMMAND: 1 },
+      env: { ...opts.env, __VERCEL_SKIP_DEV_CMD: 1 },
     }
   );
 
@@ -287,7 +287,7 @@ function testFixtureStdio(
       let printedOutput = false;
 
       const env = skipDeploy
-        ? { ...process.env, __NOW_SKIP_DEV_COMMAND: 1 }
+        ? { ...process.env, __VERCEL_SKIP_DEV_CMD: 1 }
         : process.env;
       dev = execa(
         binaryPath,