chore: exclude dist/__fixtures__ from publish

[legobeat]

Before:

$ npm publish --dry-run

> @metamask/rpc-errors@6.0.0 prepack
> ./scripts/prepack.sh

+ set -e
+ set -o pipefail
+ [[ -n '' ]]
+ yarn build
npm notice
npm notice package: @metamask/rpc-errors@6.0.0
npm notice === Tarball Contents ===
npm notice 1.1kB  LICENSE
npm notice 8.4kB  README.md
npm notice 1.3kB  dist/__fixtures__/errors.d.ts
npm notice 1.7kB  dist/__fixtures__/errors.js
npm notice 2.6kB  dist/__fixtures__/errors.js.map
npm notice 26B    dist/__fixtures__/index.d.ts
npm notice 834B   dist/__fixtures__/index.js
npm notice 194B   dist/__fixtures__/index.js.map
npm notice 1.5kB  dist/classes.d.ts
npm notice 3.6kB  dist/classes.js
npm notice 5.4kB  dist/classes.js.map
npm notice 1.8kB  dist/error-constants.d.ts
npm notice 2.7kB  dist/error-constants.js
npm notice 4.1kB  dist/error-constants.js.map
npm notice 6.9kB  dist/errors.d.ts
npm notice 8.2kB  dist/errors.js
npm notice 13.8kB dist/errors.js.map
npm notice 299B   dist/index.d.ts
npm notice 1.5kB  dist/index.js
npm notice 657B   dist/index.js.map
npm notice 2.4kB  dist/utils.d.ts
npm notice 5.3kB  dist/utils.js
npm notice 8.7kB  dist/utils.js.map
npm notice 2.4kB  package.json
npm notice === Tarball Details ===
npm notice name:          @metamask/rpc-errors
npm notice version:       6.0.0
npm notice filename:      @metamask/rpc-errors-6.0.0.tgz
npm notice package size:  18.7 kB
npm notice unpacked size: 85.4 kB
npm notice shasum:        e7ed4af4b50eb19f76ebe3f1b58db7066c9a31de
npm notice integrity:     sha512-8A7Li+sR/N0es[...]l6rRLPTkVdadQ==
npm notice total files:   24

After:

$ npm publish --dry-run

> @metamask/rpc-errors@6.0.0 prepack
> ./scripts/prepack.sh

+ set -e
+ set -o pipefail
+ [[ -n '' ]]
+ yarn build

npm notice
npm notice package: @metamask/rpc-errors@6.0.0
npm
notice === Tarball Contents ===
npm notice 1.1kB  LICENSE
npm notice 8.4kB  README.md
npm notice 1.5kB  dist/classes.d.ts
npm notice 3.6kB  dist/classes.js
npm notice 5.3kB  dist/classes.js.map
npm notice 1.8kB  dist/error-constants.d.ts
npm notice 2.7kB  dist/error-constants.js
npm notice 4.1kB  dist/error-constants.js.map
npm notice 6.7kB  dist/errors.d.ts
npm notice 8.2kB  dist/errors.js
npm notice 13.6kB dist/errors.js.map
npm notice 299B   dist/index.d.ts
npm notice 1.5kB  dist/index.js
npm notice 657B   dist/index.js.map
npm notice 2.4kB  dist/utils.d.ts
npm notice 5.3kB  dist/utils.js
npm notice 8.6kB  dist/utils.js.map
npm notice 2.4kB  package.json
npm notice === Tarball Details ===
npm notice name:          @metamask/rpc-errors
npm notice version:       6.0.0
npm notice filename:      @metamask/rpc-errors-6.0.0.tgz
npm notice package size:  16.5 kB
npm notice unpacked size: 78.2 kB
npm notice shasum:        d93562f3578d041fcb53bfaa6aac60edd899aa29
npm notice integrity:     sha512-rkUg4htJSQQq6[...]2MKssXx4QvtSA==
npm notice total files:   18

[Gudahtt]

LGTM!