Add a Mill build script

[houyuanjie]

Summary

Since playframework/play-scala-seed.g8 has been updated to support Mill 1.x, this PR introduces Mill build tool support to the current repository.

Key Changes

The build.mill script is largely identical to the upstream version, with two main differences:

1. Explicit mill-version Configuration: The mill-version is explicitly set in the build.
2. Giter8 verbatim Configuration: Instead of using phantomDeps, the build.mill file is simply added to the Giter8 verbatim list.

Rationale & Design Decisions

1. Explicit mill-version

Explicitly defining the mill-version is considered the better approach. Scala Steward should be fully capable of detecting and managing this version number automatically, just as it currently does for sbt.version.

2. Avoiding phantomDeps and $ Escaping Issues

Adding build.mill to the verbatim list bypasses the parsing confusion caused by the backslash-dollar (\$) escape sequence for both Scala Steward and Mill.
Conveniently, since build.mill now strictly accepts package as the RootModule name, we no longer need to engineer complex workarounds for Giter8 to process it dynamically.

Future Work

If this approach proves successful and stable here, these two modifications can and should be contributed back to playframework/play-scala-seed.g8.

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