Update to Mill 1.0.0 (#191)

try to use sourceFile.content again

cache based on path rather than sourcefile

cache line number and position comment parses

Update actions.yml

use new rather than apply in Scala 3 (#176)

Scala 2 macros expand to e.g. `new sourcecode.Line(2)`.

To maintain compatibility with other tools such as Mill's CodeSig
analyzer, we should directly call the constructor, rather than the
companion's `apply` method e.g. `sourcecode.Line.apply(2)`

Prepared release 0.3.1