Add support for SSE comments

[magdzikk]

Fixes #379.

ServerSentEvent gains a comments: List[String] field, so comment lines (those starting with :) round-trip through both parse and toString. ServerSentEvent.comment("ping") builds the keep-alive frame the WhatWG specification recommends sending every 15 seconds or so, to stop legacy proxies from dropping an idle connection.

Design notes

List[String] rather than Option[String], because a single event block may carry several comment lines.

There is deliberately no assertion that a comment and data aren't both set (as considered in the issue discussion). The specification ignores comments per line, not per event, so a block such as

: keep-alive
data: abc

is valid and still dispatches abc. A guard would reject valid streams.

Binary compatibility

Preserved with the same approach already used for ContentTypeRange: the old-arity constructor, copy and apply are kept alongside the new ones.

Note one source-level (not binary) change: downstream positional patterns like case ServerSentEvent(d, e, i, r) now need a fifth _. This is inherent to adding a field and is not visible to MiMa.

Migration

Comment-only blocks previously parsed to ServerSentEvent(). A keep-alive : ping now parses to ServerSentEvent(comments = List("ping")), so code that skipped such events by comparing against an empty event silently stops skipping them:

events.filter(_ != ServerSentEvent())  // no longer filters out keep-alives
events.filter(!_.isCommentOnly)        // use this instead

isCommentOnly is true when an event carries no data, event type, id or retry — only comments, if any.

Equality checks are the case to watch, because they keep compiling. The positional-pattern equivalent, case ServerSentEvent(None, None, None, None), cannot regress silently: it fails to compile until a fifth _ is added.

[adamw]

Automated review by Claude (posted by @adamw). Main finding: comments are not split on newlines in toString. Details inline.