feat(languages): .mkdn as markdown extension (helix-editor#10065)

GitHub recognizes `.mkdn` as markdown files. Add `.mkdn` as a valid
file type for Markdown.

languages.toml

@@ -1473,7 +1473,7 @@ source = { git = "https://github.com/Flakebi/tree-sitter-tablegen", rev = "568dd
 name = "markdown"
 scope = "source.md"
 injection-regex = "md|markdown"
-file-types = ["md", "markdown", "mkd", "mdwn", "mdown", "markdn", "mdtxt", "mdtext", "workbook", { glob = "PULLREQ_EDITMSG" }]
+file-types = ["md", "markdown", "mkd", "mkdn", "mdwn", "mdown", "markdn", "mdtxt", "mdtext", "workbook", { glob = "PULLREQ_EDITMSG" }]
 roots = [".marksman.toml"]
 language-servers = [ "marksman", "markdown-oxide" ]
 indent = { tab-width = 2, unit = "  " }