Comply with Gemini spec p. 5.4.1

This part of spec says soft text wrapping is preferred to hard wrapping,
to comply with that we replace the newlines in raw Markdown text with
spaces.

internal/gemini/renderer.go

@@ -22,6 +22,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"strings"
 	"time"
 
 	"github.com/gomarkdown/markdown/ast"
@@ -266,7 +267,9 @@ func (r Renderer) list(w io.Writer, node *ast.List, level int) {
 
 func (r Renderer) text(w io.Writer, node ast.Node) {
 	if node := node.AsLeaf(); node != nil {
-		w.Write(node.Literal)
+		// replace all newlines in text with spaces, allowing for soft
+		// wrapping; this is recommended as per Gemini spec p. 5.4.1
+		w.Write([]byte(strings.ReplaceAll(string(node.Literal), "\n", " ")))
 	}
 }