Add .npmignore to exclude unnecessary files into npm

[cesco69]

see https://www.npmjs.com/package/fast-json-stringify?activeTab=code

Unpacked Size: 390 kB
Total Files: 85

Name	Type	Size
.github/	folder	1.7 kB
benchmark/	folder	19.6 kB
build/	folder	702 B
examples/	folder	1.97 kB
lib/	folder	44.6 kB
test/	folder	248 kB
types/	folder	11.6 kB
.gitattributes	text/plain	80 B
LICENSE	text/plain	1.2 kB
README.md	text/markdown	22.4 kB
eslint.config.js	application/javascript	179 B
index.js	application/javascript	35.2 kB
package.json	application/json	2.31 kB

This PR cleanup in

Name	Type	Size
build/	folder	702 B
lib/	folder	44.6 kB
types/	folder	11.6 kB
LICENSE	text/plain	1.2 kB
README.md	text/markdown	22.4 kB
index.js	application/javascript	35.2 kB
package.json	application/json	2.31 kB

[jean-michelet]

This is somewhat controversial, but I think we always publish docs, tests etc on npm.
The reason might be that if the repository suddenly disappear, the user can still maintain the code.

[cesco69]

ok, feel free to close the PR if not necessary!

[jean-michelet]

Yes, two core devs have confirmed my point.
But thanks for wanting to help!