Exclude files from dist package

[cedric-anne]

For the time being, when fetching the dist package from composer, all repository files are included. Using export-ignore git attribute would permit to make the dist package ligther (see https://php.watch/articles/composer-gitattributes#export-ignore).

Ignore files weight: 68.9kB Remaining files weight: 28.3kB

[donatj]

Is this worth the ongoing maintenance cost for 68kb?