Added one click install command feature

[ebenjs]

This PR is targeting issue #7122

We're used to having the ability to easily install packages by copying the install code and pasting it directly into the console. This is especially helpful when you need to quickly install a package. Popular platforms like https://www.npmjs.com/package/axios, https://crates.io/crates/http, and https://pypi.org/project/http1/ all employ this design.

Currently, there's something similar in the header section of the package page after the package name. However, this command only allows you to copy text that's intended to be pasted in the pubspec.yml file and manually run 'pub get install.'

Therefore, I suggest adding this feature as it would be a time-saving feature for all developers.

Here is a screenshot

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[isoos]

Note: I've pushed this to the staging site:
https://isoos-dot-dartlang-pub-dev.appspot.com/packages/email_validator

I find the gray background a bit off in the sidebar, I think we should look for a more blended-in design.

/cc @jonasfj @szakarias @sigurdm