Avoid packaging extraneous files when publishing to npm

When publishing npm packages for the first time during the release of 18.0.0RC1 we notice that multiple files and directories were added to the packages but are not strictly needed for the good reuse of the packages by other projects.
This includes:

• temp directory
• build files such as tsconfig.json files
• test sources

After closer inspection, what's strictly needed is:

• the src directory stripped of its test souces
• the dist directory exhaustivelly
• the package.json file
• a README file (will be handled in a separate issue), to have a nicer experience when browsing the packages on https://www.npmjs.com/