[15-min-fix] Add option for triple tildes for fenced codeblocks (#13810)

* Add option for triple tildes for fenced codeblocks

Note that this doesn't account for all triple backticks in fenced codeblocks situations.

* Add commented out but failing test case for the future?

* Add tests from QA instructions

app/services/markdown_processor/parser.rb

@@ -114,7 +114,7 @@ module MarkdownProcessor
 
     def escape_liquid_tags_in_codeblock(content)
       # Escape codeblocks, code spans, and inline code
-      content.gsub(/[[:space:]]*`{3}.*?`{3}|`{2}.+?`{2}|`{1}.+?`{1}/m) do |codeblock|
+      content.gsub(/[[:space:]]*~{3}.*?~{3}|[[:space:]]*`{3}.*?`{3}|`{2}.+?`{2}|`{1}.+?`{1}/m) do |codeblock|
         codeblock.gsub!("{% endraw %}", "{----% endraw %----}")
         codeblock.gsub!("{% raw %}", "{----% raw %----}")
         if codeblock.match?(/[[:space:]]*`{3}/)

spec/services/markdown_processor/parser_spec.rb

@@ -27,6 +27,19 @@ RSpec.describe MarkdownProcessor::Parser, type: :service do
     expect(generate_and_parse_markdown(code_block)).not_to include("----")
   end
 
+  it "escapes some triple backticks within a codeblock when using tildes" do
+    code_block = "​~~~\nhello\n// ```\nwhatever\n// ```\n~~~"
+    number_of_triple_backticks = generate_and_parse_markdown(code_block).scan("```").count
+    expect(number_of_triple_backticks).to eq(2)
+  end
+
+  # TODO: @zhao-andy this should fail if this issue is solved: https://github.com/forem/forem/issues/13823
+  it "escapes triple backticks within a codeblock when using tildes" do
+    code_block = "~~~\nhello\n```\nwhatever\n```\n~~~"
+    number_of_triple_backticks = generate_and_parse_markdown(code_block).scan("```").count
+    expect(number_of_triple_backticks).to eq(0)
+  end
+
   it "does not remove the non-'raw tag related' four dashes" do
     code_block = "```\n----\n```"
     expect(generate_and_parse_markdown(code_block)).to include("----")