fix: prevent context fetcherOptions being overwritten by mutate

[dan-cooke]

Fixes #226

[dan-cooke]

I am having difficulties updating the snapshots locally, I am getting strange jest runtime errors

Error: You need to run with a version of node that supports ES Modules in the VM API. See https://jestjs.io/docs/ecmascript-modules

If anyone else is able to pull this branch, update the snapshots and merge this PR in that would be great

[dan-cooke]

I'm also unsure about the dependency on lodash/merge , can probably copy it into the repo actually

[Cellule]

To update snapshots you have 2 options, wait on #234
Or simpler, temporarily downgrade pretter to v2, update the snapshot then revert prettier back to v3