feat(cli): add 'coder login token' command to print session token (#21627)

Adds a new subcommand to print the current session token for use in
scripts and automation, similar to `gh auth token`.

## Usage

```bash
CODER_SESSION_TOKEN=$(coder login token)
```

Fixes #21515

cli/testdata/coder_login_--help.golden

@@ -9,6 +9,9 @@ USAGE:
   macOS and Windows and a plain text file on Linux. Use the --use-keyring flag
   or CODER_USE_KEYRING environment variable to change the storage mechanism.
 
+SUBCOMMANDS:
+    token    Print the current session token
+
 OPTIONS:
       --first-user-email string, $CODER_FIRST_USER_EMAIL
           Specifies an email address to use if creating the first user for the

cli/testdata/coder_login_token_--help.golden

@@ -0,0 +1,11 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder login token
+
+  Print the current session token
+
+  Print the session token for use in scripts and automation.
+
+———
+Run `coder --help` for a list of global options.