fix: replace lodash.merge with just-extend

[osdiab]

This replaces lodash.merge with just-extend to avoid issues where Vercel's Edge Runtime would crash if you used extendDictionary because dynamic code execution is not supported on the edge. just-extend does not appear to do this so in theory it should be OK.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time.
  • Using typesafe-i18n with Next.js 13 App Directory #580 (reply in thread)
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
  • i wrote a test, it passes either way. not sure if i did it right or if you'd like more rigorous tests, though.
  • i have not tested this in the edge runtime, nor have i tested this on more complex use-cases. but the existing tests seem to be happy.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint

[ivanhofer]

Thanks for the PR