Switch Unicode Escaping to a VSCode-like system (#19990)

This PR rewrites the invisible unicode detection algorithm to more
closely match that of the Monaco editor on the system. It provides a
technique for detecting ambiguous characters and relaxes the detection
of combining marks.

Control characters are in addition detected as invisible in this
implementation whereas they are not on monaco but this is related to
font issues.

Close #19913

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/charset/escape_status.go

@@ -0,0 +1,28 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package charset
+
+// EscapeStatus represents the findings of the unicode escaper
+type EscapeStatus struct {
+	Escaped      bool
+	HasError     bool
+	HasBadRunes  bool
+	HasInvisible bool
+	HasAmbiguous bool
+}
+
+// Or combines two EscapeStatus structs into one representing the conjunction of the two
+func (status *EscapeStatus) Or(other *EscapeStatus) *EscapeStatus {
+	st := status
+	if status == nil {
+		st = &EscapeStatus{}
+	}
+	st.Escaped = st.Escaped || other.Escaped
+	st.HasError = st.HasError || other.HasError
+	st.HasBadRunes = st.HasBadRunes || other.HasBadRunes
+	st.HasAmbiguous = st.HasAmbiguous || other.HasAmbiguous
+	st.HasInvisible = st.HasInvisible || other.HasInvisible
+	return st
+}