Adjust isMarkdownFile heuristics to not Indirectly Depend Upon Language Extensions

[farmerau]

Description

Per #219, isMarkdownFile relies on language support for MDX files to be installed in order to work. This change adds fallback logic to the method to support checking the supported file extensions (md, mdx) to support these file types without need for users to install MDX language support.

Testing

To test the changes, I:

1. Ran the extension using the Extension Development Host
2. Ensured that MDX language support was disabled
3. Ensured that frontMatter.content.autoUpdateDate was set to true
4. Opened a workspace that had MDX files
5. Edited an MDX file and observed that lastmod property was updated in Front Matter as expected.

Notes:

As discussed in the issue, there's an alternative path for resolving this: explicitly requiring mdx language support in the extension manifest. This might not be desirable-- since it's unknown (to me) how many people are using Front Matter for MDX instead of Markdown.

[estruyf]

Thanks @farmerau! This is very much appreciated 🙏