RemoveUnused: detect unused function parameters in Scala 3

[bjaglin]

Despite the presence of -Wunused:params and the explicit mention of it in scala/scala3#16157, I did not manage to get diagnostics (nor warnings) for unused parameters with scala 3.4.0-RC2, so the feature is currently documented/tested as Scala2-only.

We probably need to report this upstream.

$ metap ./scalafix-tests/input/target/jvm-3.4.0-RC2/meta/META-INF/semanticdb/scalafix-tests/input/src/main/scala/test/removeUnused/RemoveUnusedParams.scala.semanticdb 
scalafix-tests/input/src/main/scala/test/removeUnused/RemoveUnusedParams.scala
------------------------------------------------------------------------------

Summary:
Schema => SemanticDB v4
Uri => scalafix-tests/input/src/main/scala/test/removeUnused/RemoveUnusedParams.scala
Text => empty
Language => Scala
Symbols => 11 entries
Occurrences => 28 entries

Symbols:
local0 => param unused: String
local1 => param unused: String
local2 => param used: String
local3 => param unused: Long
local4 => implicit param string: String
test/removeUnused/UnusedParams. => final object UnusedParams extends Object { self: UnusedParams.type => +5 decls }
test/removeUnused/UnusedParams.f. => val method fFunction1[String, Unit]
test/removeUnused/UnusedParams.ff. => val method ffFunction1[String, Unit]
test/removeUnused/UnusedParams.fs. => val method fsFunction2[String, Long, Unit]
test/removeUnused/UnusedParams.g(). => method g(x: Function1[String, Unit]): Unit
test/removeUnused/UnusedParams.g().(x) => param x: Function1[String, Unit]

Occurrences:
[3:8..3:12) => test/
[3:13..3:25) <= test/removeUnused/
[5:7..5:19) <= test/removeUnused/UnusedParams.
[6:6..6:7) <= test/removeUnused/UnusedParams.f.
[6:9..6:15) => scala/Predef.String#
[6:19..6:23) => scala/Unit#
[6:26..6:32) <= local0
[6:36..6:43) => scala/Predef.println(+1).
[7:6..7:8) <= test/removeUnused/UnusedParams.ff.
[7:12..7:18) <= local1
[7:20..7:26) => scala/Predef.String#
[7:31..7:38) => scala/Predef.println(+1).
[8:6..8:8) <= test/removeUnused/UnusedParams.fs.
[8:12..8:16) <= local2
[8:18..8:24) => scala/Predef.String#
[8:26..8:32) <= local3
[8:34..8:38) => scala/Long#
[8:43..8:50) => scala/Predef.println(+1).
[8:51..8:55) => local2
[9:6..9:7) <= test/removeUnused/UnusedParams.g().
[9:8..9:9) <= test/removeUnused/UnusedParams.g().(x)
[9:11..9:17) => scala/Predef.String#
[9:21..9:25) => scala/Unit#
[9:28..9:32) => scala/Unit#
[9:35..9:38) => scala/Predef.`???`().
[10:2..10:3) => test/removeUnused/UnusedParams.g().
[10:13..10:19) <= local4
[10:23..10:30) => scala/Predef.println(+1).

Originally posted by @bjaglin in #1728 (comment)