Small Gradle plugin QoL improvements

[ZacSweers]

Just a few small things I thought I'd toss up for consideration

• Add @SqlDelightDsl DSL marker. This prevents accidentally nesting multiple sqldelight DSL calls.
• Use project.objects for initializing properties with conventions, allowing SqlDelightDatabase to avoid "accessing non-final property in constructor" warnings.
• Use detachedConfiguration(), which is functionally achieving the same as before but a first-party API from gradle for accomplishing the same thing (i.e. plugin-internal configurations).

[ZacSweers]

Removing the detachedConfiguration stuff because, to the surprise of no one, it works differently than expected and has no docs or plans to make it better.

[hfhbd]

Does project.objects have any effect besides the (wrong) compiler message? I think this is just a style preference that I like more.
Regarding @SqlDelightDsl, interesting idea, never seen it before in Gradle plugins. This would only affect Kotlin DSL scripts, won't it?

[ZacSweers]

What makes you say the compiler is wrong?

Re: the DSL, yes it would only work in kotlin build scripts