Clean up fansi.Str constructors

[lihaoyi]

• Remove case modifier from fansi.Str: we're already overriding apply, equals, hashcode, etc. so being a case class buys us nothing.

• Rename fansi.Str.join to fansi.Str.apply, to provide an easy way of concatenating a bunch of string literals e.g. fansi.Str("hello", "world", "i am cow")

• Added a new fansi.Str.join method that takes a Seq[fansi.Str], to make it a bit more ergonomic to join such Seqs together. Also include an optional sep argument, to handle the use case that mkString satisfies on Seq[String]

This is a breaking change, so since we're already breaking compat in a few places in com-lihaoyi we should include this in the update

Review by @lolgab