Upgrade for current Go releases.

Requires go 1.26.0 (golang.org/x/tools v0.50.0 is needed to read export
data from modern toolchains).

Behaviour changes:
- A run of two or more blank lines now round-trips as a single blank
  line, matching gofmt. Previously it was dropped entirely (#81).
- File.GoVersion is preserved through decorate and restore.