Add "markdown" as a possible filetype (helix-editor#3749)

Author: Akselmo

languages.toml

@@ -904,7 +904,7 @@ source = { git = "https://github.com/Flakebi/tree-sitter-tablegen", rev = "568dd
 name = "markdown"
 scope = "source.md"
 injection-regex = "md|markdown"
-file-types = ["md"]
+file-types = ["md", "markdown"]
 roots = []
 indent = { tab-width = 2, unit = "  " }