Do not try flatparse scanner for .md files

[janmasrovira]

• In Fix #2924. Use MegaParsec scanner for Markdown files #2925, the flatparse scanner always fails for .md files and silently falls back to megaparsec.
• In Fix scanning of names with import prefix #2929, a warning is introduced that informs the user when the fallback parser is being used. This is causing a warning every time we scan a markdown file.
• This pr fixes the problem by changing the default strategy to directly use megaparsec when a .md file is given.