feat: adding Fr back as a BB export (ts)

[signorecello]

Smallest PR ever. closes #2369

Fr stopped being exported since bb.js 0.0.1, and it's a very useful util class for getting field values from strings, buffers, or to strings and buffers.

We've been getting requests for this from Noir developers, and devrel actually needs this in order to refactor the Stealthdrop example.

Checklist:

Remove the checklist to signal you've completed it. Enable auto-merge if the PR is ready to merge.

• If the pull request requires a cryptography review (e.g. cryptographic algorithm implementations) I have added the 'crypto' tag.
• I have reviewed my diff in github, line by line and removed unexpected formatting changes, testing logs, or commented-out code.
• Every change is related to the PR description.
• I have linked this pull request to relevant issues (if any exist).