Remove wxUSE_STL which is not really used any longer

wxString is always based on std::[w]string since 2c0c727f49 (Remove wx
own wxStringImpl implementation, 2022-11-16) and all containers use
standard containers by default too now -- and there is a separate
wxUSE_STD_CONTAINERS for this anyhow.

The only remaining use of wxUSE_STL was as the default value for
wxUSE_STD_STRING_CONV_IN_WXSTRING option, but it's not really needed
for this neither, and this option can just be set to 0 by default.

Also add wxUSE_CHAR_CONV_IN_WXSTRING which can now be set to 0 too to
disable all unwanted implicit conversions (even "safe" ones, to wide
strings, in addition to the unsafe ones to narrow strings that could be
already disabled with wxUSE_UNSAFE_WXSTRING_CONV) to allow people who
don't want to have any implicit conversions at all to do it.

Keep --enable-stl configure option for compatibility, but warn if it is
used to tell people that it is not needed any longer.

appveyor.yml

@@ -33,7 +33,6 @@ environment:
   - TOOLSET: msbuild
     CONFIGURATION: DLL Release
     ARCH: x64
-    wxUSE_STL: 1
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2022
   - TOOLSET: msbuild
     CONFIGURATION: Debug
@@ -44,7 +43,6 @@ environment:
     VS: '14.0'
     BUILD: debug
     ARCH: amd64
-    wxUSE_STL: 1
     wxUSE_WEBVIEW_EDGE: 1
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
   - TOOLSET: cmake
@@ -72,13 +70,11 @@ init:
 before_build:
 - ps: |
     $env:PATH = $env:PATH -replace "C:\\Program Files\\Git\\usr\\bin",""
-    if (-not (Test-Path env:wxUSE_STL)) { $env:wxUSE_STL = '0' }
     if (-not (Test-Path env:wxUSE_UNICODE_UTF8)) { $env:wxUSE_UNICODE_UTF8 = '0' }
     if (-not (Test-Path env:wxUSE_WEBVIEW_EDGE)) { $env:wxUSE_WEBVIEW_EDGE = '0' }
     if (($env:TOOLSET -ne "msys2") -and ($env:TOOLSET -ne "cygwin")) {
       $txt = gc include\wx\msw\setup.h
       Write-Output $txt |
-      %{$_ -replace "define wxUSE_STL 0", "define wxUSE_STL $env:wxUSE_STL"} |
       %{$_ -replace "define wxUSE_UNICODE_UTF8 0", "define wxUSE_UNICODE_UTF8 $env:wxUSE_UNICODE_UTF8"} |
       %{$_ -replace "define wxUSE_WEBVIEW_EDGE 0", "define wxUSE_WEBVIEW_EDGE $env:wxUSE_WEBVIEW_EDGE"} |
       sc include\wx\msw\setup.h