sbt 2.x support for `laika-sbt`

[tanishiking]

sbt-typelevel depends on laika-sbt typelevel/sbt-typelevel#899

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I'm working on cross-building for sbt 2.x, and I found sbt 2.0.0 has a macro crash when a Def.setting or Def.task body contains a generic method call with a HKT type argument. Laika hits this through its IO-based builder APIs, so sbt 2 support may need an upstream sbt fix sbt/sbt#9375

[tanishiking]

sbt 2.0.1 has released, and now #774 is ready for review ;)

[eshu]

How to vote for this issue? :)