d2-vscode LanguageServerChanges

These changes are for the language server in d2-vscode.

When the D2_LSP_MODE environment variable is set, the
d2 cli will read the d2 file, produce the ast (and possible
errors), convert it to JSON, print it out to stdout,
then terminate.  This was done this way to keep the
changes to the d2 cli code to a minimum.

PR for d2-vscode to come after this is accepted

d2compiler/compile.go

@@ -1,11 +1,13 @@
 package d2compiler
 
 import (
+	"encoding/json"
 	"encoding/xml"
 	"fmt"
 	"io"
 	"io/fs"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 
@@ -27,18 +29,46 @@ type CompileOptions struct {
 	FS fs.FS
 }
 
+// Changes for Language Server 'mode'
+type LspOutputData struct {
+	Ast *d2ast.Map
+	Err error
+}
+
+var lod LspOutputData
+
+func LspOutput(m bool) {
+	if !m {
+		return
+	}
+
+	jsonOutput, _ := json.Marshal(lod)
+	fmt.Print(string(jsonOutput))
+	os.Exit(42)
+}
+
 func Compile(p string, r io.Reader, opts *CompileOptions) (*d2graph.Graph, *d2target.Config, error) {
 	if opts == nil {
 		opts = &CompileOptions{}
 	}
 
+	lspMode := os.Getenv("D2_LSP_MODE") == "1"
+	defer LspOutput(lspMode)
+
 	ast, err := d2parser.Parse(p, r, &d2parser.ParseOptions{
 		UTF16Pos: opts.UTF16Pos,
 	})
+
+	lod.Ast = ast
 	if err != nil {
+		lod.Err = err
 		return nil, nil, err
 	}
 
+	if lspMode {
+		return nil, nil, nil
+	}
+
 	ir, err := d2ir.Compile(ast, &d2ir.CompileOptions{
 		UTF16Pos: opts.UTF16Pos,
 		FS:       opts.FS,