to-do: trim @std/io APIs

[iuioiua]

@std/io contains quite a few deprecated APIs. We should review each one, and if still relevant, provide a migration guide, deprecate, then remove. Doing so pushes users to use Streams APIs and @std/streams, and reduces the number of API choices they need to make for their projects.

Symbol	Deprecated	Removal PR	Removed
BufReader	✅	#6056	✅
BufWriter	✅	#6057	✅
copyN()	✅	#5999	✅
LimitedReader	✅	#6058	✅
MultiReader	✅	#6059	✅
readDelim()	✅	#6052	✅
readInt()	✅	#6048	✅
readLines()	✅	#5991	✅
readLong()	✅	#6047	✅
readRange[Sync]()	✅	#6049	✅
readShort()	✅	#6050	✅
readStringDelim()	✅	#6001	✅
sliceLongToBytes()	✅	#6005	✅
StringReader	✅	#6062	✅
StringWriter	✅	#6061	✅