Change markdown rendering from blackfriday to goldmark (#9533)

* Move to goldmark

Markdown rendering moved from blackfriday to the goldmark.

Multiple subtle changes required to the goldmark extensions to keep
current rendering and defaults.

Can go further with goldmark linkify and have this work within markdown
rendering making the link processor unnecessary.

Need to think about how to go about allowing extensions - at present it
seems that these would be hard to do without recompilation.

* linter fixes

Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/yuin/goldmark/parser/blockquote.go

@@ -0,0 +1,69 @@
+package parser
+
+import (
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+type blockquoteParser struct {
+}
+
+var defaultBlockquoteParser = &blockquoteParser{}
+
+// NewBlockquoteParser returns a new BlockParser that
+// parses blockquotes.
+func NewBlockquoteParser() BlockParser {
+	return defaultBlockquoteParser
+}
+
+func (b *blockquoteParser) process(reader text.Reader) bool {
+	line, _ := reader.PeekLine()
+	w, pos := util.IndentWidth(line, reader.LineOffset())
+	if w > 3 || pos >= len(line) || line[pos] != '>' {
+		return false
+	}
+	pos++
+	if pos >= len(line) || line[pos] == '\n' {
+		reader.Advance(pos)
+		return true
+	}
+	if line[pos] == ' ' || line[pos] == '\t' {
+		pos++
+	}
+	reader.Advance(pos)
+	if line[pos-1] == '\t' {
+		reader.SetPadding(2)
+	}
+	return true
+}
+
+func (b *blockquoteParser) Trigger() []byte {
+	return []byte{'>'}
+}
+
+func (b *blockquoteParser) Open(parent ast.Node, reader text.Reader, pc Context) (ast.Node, State) {
+	if b.process(reader) {
+		return ast.NewBlockquote(), HasChildren
+	}
+	return nil, NoChildren
+}
+
+func (b *blockquoteParser) Continue(node ast.Node, reader text.Reader, pc Context) State {
+	if b.process(reader) {
+		return Continue | HasChildren
+	}
+	return Close
+}
+
+func (b *blockquoteParser) Close(node ast.Node, reader text.Reader, pc Context) {
+	// nothing to do
+}
+
+func (b *blockquoteParser) CanInterruptParagraph() bool {
+	return true
+}
+
+func (b *blockquoteParser) CanAcceptIndentedLine() bool {
+	return false
+}