[v4] [icons] fix: remove dependency on type-fest

[adidahiya]

Changes proposed in this pull request:

Remove dependency on "type-fest" package since it causes some surprising TS errors downstream:

[error] TypeScript error node_modules/.pnpm/type-fest@1.4.0/node_modules/type-fest/ts41/get.d.ts:68:45 - error TS2344: Type 'string | number' does not satisfy the constraint 'string'.
  Type 'number' is not assignable to type 'string'.

68 type WithStringKeys<BaseType extends Record<string | number, any>> = {
                                               ~~~~~~~~~~~~~~~

In its place, I've copy-pasted the code into the icons package. type-fest's README says this is ok to do, no attribution needed.

[blueprint-bot]

fix file ext

Previews: documentation | landing | table | modern colors demo

[blueprint-bot]

fix lint

Previews: documentation | landing | table | modern colors demo