Add tailwindcss-rails as runtime dep for solidus_dev_support

[mamhoff]

This way, all extensions will have tailwind as their dev dependency.

[mergify[bot]]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.

[tvdeyen]

This way, all extensions will have tailwind as their dev dependency.

Of course they will have it, once we add it 😏. But the question is why do all extensions need tailwind?

[mamhoff]

I don't actually remember why I put this up. Closing.