implement Phoenix.HTML.Safe protocol

[hdahlheim]

Adds a struct for the PhoenixImportmap.Importmap module that implements the Phoenix.HTML.Safe protocol discussed in #4.

By using the protocol, we signal to the user that the return value of PhoenixImportmap.importmap/1 is safe to inject into the template.

Changes made in this PR

• Adds :phoenix_html as a dependency.
• Adds a struct to the PhoenixImportmap.Importmap module.
• Derives Jason.Encoder for %PhoenixImportmap.Importmap{}.
• Removes Importmap.json/1 from PhoenixImportmap.importmap/1.
• Updates the test for PhoenixImportmap.importmap/1 to check for safe serialization.
• Updates relevant documentation to reflect these changes.

This change is a breaking change if you currently use raw to interpolate the importmap into the template.

Let me know if these changes look good to you.

[gilest]

Looks great, and CI passes ✅

If you could please correct the doc typo I think we can merge and release.

For now I'll create a manual changelog since I don't have release automation set up yet