fix: tilde expansion for aliases

[gregnr]

Tilde expansion for aliases currently fails:

const bash = new Bash({
  env: { HOME: "/home/user", BASH_ALIAS_ll: "ls -alF" },
});
await bash.exec("shopt -s expand_aliases");

await bash.exec("ls -alF ~/"); // lists /home/user
await bash.exec("ll ~/");      // lists /

Problem was that wordNodeToString() in alias-expansion.ts serializes alias args back to a string, but was missing a case for TildeExpansion nodes so the ~ got silently dropped. This PR adds the TildeExpansion case + a test to confirm the fix.

[vercel]

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