[MM-11143] Strip markdown formatting characters from push notifications (#17775)

* WIP: trim markdown for push notification

* fix: golangci

* fix: table regex

* fix: regex code block

* doc: add license

* WIP: custom renderer with goldmark

* WIP: update goldmark version to 1.38

* fix: use goldmark as parser

* fix: remove table extension

* fix: change buf to `strings.Builder`

* fix: return original string, log warning if error

* refactor: change `WriteString` to `WriteByte`

* refactor: change if condition

* refactor: use assertion

* refactor: move to inline

* fix: remove handle multiline

Already handled by mobile

* refactor: wrap same function

* refactor: move strip markdown to `sendPushNotificationSync`

* refactor: renaming variable aren't used

* fix: move log to func `sendPushNotificationSync`

* docs: add comment to `StripMarkdown`

* fix: move assign message to else

* appErr to err rename

Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

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

@@ -0,0 +1,96 @@
+package parser
+
+import (
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+type codeBlockParser struct {
+}
+
+// CodeBlockParser is a BlockParser implementation that parses indented code blocks.
+var defaultCodeBlockParser = &codeBlockParser{}
+
+// NewCodeBlockParser returns a new BlockParser that
+// parses code blocks.
+func NewCodeBlockParser() BlockParser {
+	return defaultCodeBlockParser
+}
+
+func (b *codeBlockParser) Trigger() []byte {
+	return nil
+}
+
+func (b *codeBlockParser) Open(parent ast.Node, reader text.Reader, pc Context) (ast.Node, State) {
+	line, segment := reader.PeekLine()
+	pos, padding := util.IndentPosition(line, reader.LineOffset(), 4)
+	if pos < 0 || util.IsBlank(line) {
+		return nil, NoChildren
+	}
+	node := ast.NewCodeBlock()
+	reader.AdvanceAndSetPadding(pos, padding)
+	_, segment = reader.PeekLine()
+	node.Lines().Append(segment)
+	reader.Advance(segment.Len() - 1)
+	return node, NoChildren
+
+}
+
+func (b *codeBlockParser) Continue(node ast.Node, reader text.Reader, pc Context) State {
+	line, segment := reader.PeekLine()
+	if util.IsBlank(line) {
+		node.Lines().Append(segment.TrimLeftSpaceWidth(4, reader.Source()))
+		return Continue | NoChildren
+	}
+	pos, padding := util.IndentPosition(line, reader.LineOffset(), 4)
+	if pos < 0 {
+		return Close
+	}
+	reader.AdvanceAndSetPadding(pos, padding)
+	_, segment = reader.PeekLine()
+
+	// if code block line starts with a tab, keep a tab as it is.
+	if segment.Padding != 0 {
+		preserveLeadingTabInCodeBlock(&segment, reader, 0)
+	}
+
+	node.Lines().Append(segment)
+	reader.Advance(segment.Len() - 1)
+	return Continue | NoChildren
+}
+
+func (b *codeBlockParser) Close(node ast.Node, reader text.Reader, pc Context) {
+	// trim trailing blank lines
+	lines := node.Lines()
+	length := lines.Len() - 1
+	source := reader.Source()
+	for length >= 0 {
+		line := lines.At(length)
+		if util.IsBlank(line.Value(source)) {
+			length--
+		} else {
+			break
+		}
+	}
+	lines.SetSliced(0, length+1)
+}
+
+func (b *codeBlockParser) CanInterruptParagraph() bool {
+	return false
+}
+
+func (b *codeBlockParser) CanAcceptIndentedLine() bool {
+	return true
+}
+
+func preserveLeadingTabInCodeBlock(segment *text.Segment, reader text.Reader, indent int) {
+	offsetWithPadding := reader.LineOffset() + indent
+	sl, ss := reader.Position()
+	reader.SetPosition(sl, text.NewSegment(ss.Start-1, ss.Stop))
+	if offsetWithPadding == reader.LineOffset() {
+		segment.Padding = 0
+		segment.Start--
+	}
+	reader.SetPosition(sl, ss)
+}