Update flexsearch client-side dependency

[terror]

Tried to upgrade this in #594

From https://github.com/terror/mcgill.courses/actions/runs/10017418575/job/27691578205

"Index" is not exported by "node_modules/.pnpm/flexsearch@0.7.43/node_modules/flexsearch/dist/flexsearch.bundle.module.min.js", imported by "client/src/lib/searchIndex.ts".
file: /Users/runner/work/mcgill.courses/mcgill.courses/client/src/lib/searchIndex.ts:1:9
1: import { Index } from 'flexsearch';
            ^
2: import _ from 'lodash';
✓ built in 5.[21](https://github.com/terror/mcgill.courses/actions/runs/10017418575/job/27691578205?pr=594#step:6:22)s
error during build:
RollupError: "Index" is not exported by "node_modules/.pnpm/flexsearch@0.7.43/node_modules/flexsearch/dist/flexsearch.bundle.module.min.js", imported by "client/src/lib/searchIndex.ts".