Update dart language server start command

Update the command to follow latest.
Please see: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/tool/lsp_spec/README.md

README.md

@@ -294,8 +294,7 @@ let g:lsp_settings = {
     \ 'analysis-server-dart-snapshot': {
     \     'cmd': [
     \         '/path/to/your/dart-sdk/bin/dart',
-    \         '/path/to/your/dart-sdk/bin/bin/snapshots/analysis_server.dart.snapshot',
-    \         '--lsp'
+    \         'language-server'
     \     ],
     \ },
 \ }

installer/install-analysis-server-dart-snapshot.cmd

@@ -6,6 +6,6 @@ del dartsdk-windows-x64-release.zip
 
 echo @echo off ^
 
-%%~dp0\dart-sdk\bin\dart %%~dp0\dart-sdk\bin\snapshots\analysis_server.dart.snapshot --lsp %%* ^
+%%~dp0\dart-sdk\bin\dart language-server %%* ^
 
 > analysis-server-dart-snapshot.cmd

installer/install-analysis-server-dart-snapshot.sh

@@ -23,7 +23,7 @@ cat <<EOF >analysis-server-dart-snapshot
 #!/usr/bin/env bash
 
 DIR=\$(cd \$(dirname \$0); pwd)/dart-sdk
-\$DIR/bin/dart \$DIR/bin/snapshots/analysis_server.dart.snapshot --lsp \$*
+\$DIR/bin/dart language-server \$*
 EOF
 
 chmod +x analysis-server-dart-snapshot