fix(stdlib): use first-element flag in $ for collections (#18583) (#25832)

Fixes #18583.

## Problem

Several stdlib collection types compute the separator for `$` using
`result.len > 1`, where `result` starts as the opening bracket (`"["` or
`"{"`). This breaks when a collection element type has a `$` that
returns an empty string: `result.len` stays at 1 after the first item
contributes nothing, so the separator is never inserted for subsequent
items.

```nim
import std/deques

type Test = object
proc `$`(x: Test): string = ""

echo [Test(), Test()].toDeque  # prints [] — expected [, ]
```

## Fix

Replace the length check with an explicit `first` flag in all affected
modules: `deques`, `heapqueue`, `lists`, `critbits`, and `strtabs`.

## Tests

Regression tests added to `tdeques`, `theapqueue`, and `tlists` using a
local type whose `$` returns `""`. All three test files pass with `nim c
-r`.

## Notes

I work with Claude as a co-processor. I'm 56, came to programming late,
and this is genuinely how I learn and contribute. I understand what I'm
submitting, but I didn't write it alone. If your project prefers
human-only contributions, just say so and I'll close without friction.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/stdlib/tdeques.nim

@@ -241,3 +241,12 @@ proc main() =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+# $ separator must be emitted even when the item's string repr is empty
+type EmptyStr18583 = object
+proc `$`(x: EmptyStr18583): string = ""
+
+block:
+  var d = [EmptyStr18583(), EmptyStr18583()].toDeque
+  doAssert $d == "[, ]", "got: " & $d

tests/stdlib/theapqueue.nim

@@ -104,3 +104,15 @@ template main() =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+type EmptyStr18583HeapQ = object
+proc `$`(x: EmptyStr18583HeapQ): string = ""
+proc `<`(a, b: EmptyStr18583HeapQ): bool = false
+
+block:
+  var h = initHeapQueue[EmptyStr18583HeapQ]()
+  push(h, EmptyStr18583HeapQ())
+  push(h, EmptyStr18583HeapQ())
+  let s = $h
+  doAssert s == "[, ]", "got: " & s

tests/stdlib/tlists.nim

@@ -287,3 +287,14 @@ template main =
 
 static: main()
 main()
+
+# https://github.com/nim-lang/Nim/issues/18583
+type EmptyStr18583List = object
+proc `$`(x: EmptyStr18583List): string = ""
+
+block:
+  var L: SinglyLinkedList[EmptyStr18583List]
+  L.prepend(EmptyStr18583List())
+  L.prepend(EmptyStr18583List())
+  let s = $L
+  doAssert s == "[, ]", "got: " & s