feat: live LSP error underlines + improved VS Code extension

bux-lsp 0.17 publishes in-process diagnostics on open/change/save so
editors show red squiggles for lex/parse/type errors in the live buffer.
VS Code client discovers tools/bux-lsp, adds status bar/restart, richer
syntax/snippets, and docs (docs/LSP.md) for forum/editor setup.

vscode/.vscodeignore

@@ -0,0 +1,15 @@
+.vscode/**
+.vscode-test/**
+src/**
+.gitignore
+.vscodeignore
+tsconfig.json
+**/*.ts
+**/*.map
+node_modules/**
+!node_modules/vscode-languageclient/**
+!node_modules/vscode-jsonrpc/**
+!node_modules/vscode-languageserver-protocol/**
+!node_modules/vscode-languageserver-types/**
+!node_modules/semver/**
+*.vsix