Allow configuring an installation script

[alanorozco]

In some environments, an installation script must be specified to limit the amount of dependencies.

We have a monorepo in our case, in which the build dependencies are scoped to different packages.

As a result, npm ci is much slower than our custom build-only install script. A full install takes about ~60s, whereas a build-only install takes about ~5s. Our builds take less than 5s, so the ability to configure install would reduce our total time to run compressed-size-action from ~130s to less than 20s.

[alanorozco]

I see now that #85 attempts the same thing in a slightly different way.

Would love to hear feedback about the overall feature. Either of the two changes would be a huge boon for us! :)

[nicklmart]

also have a need for this, would like to run npm ci --ignore-scripts instead of npm ci

[xaviermariaconejo]

Also need this here. I haven't seen this PR and I created another one 😂 https://github.com/preactjs/compressed-size-action/pull/99

¿What was your solution guys?

[rschristian]

Closing in favor of #85

Edit: I did largely copy your docs over though, thanks for writing them!