ft_func: translate treesitter-language to filetypes (close #1174).

Sometimes the treesitter-language-name coincides with the filetype (like for cpp), but in other cases, these are wholly different (lang: latex, filetype: tex/sty/cls). So, to be more accomodating and forego the need on the users part to define the correct `ls.filetype_extend`, we resolve language to filetypes in here.

doc/luasnip.txt

@@ -1,4 +1,4 @@
-*luasnip.txt*             For NVIM v0.8.0             Last change: 2024 May 16
+*luasnip.txt*             For NVIM v0.8.0             Last change: 2024 May 17
 
 ==============================================================================
 Table of Contents                                  *luasnip-table-of-contents*

lua/luasnip/extras/filetype_functions.lua

@@ -1,27 +1,39 @@
+local function fts_from_ts_lang(lang)
+	local fts = {}
+	-- In case of someone using nvim <= 0.9
+	if vim.treesitter.language and vim.treesitter.language.get_filetypes then
+		fts = vim.treesitter.language.get_filetypes(lang)
+	end
+	-- Keep lang as part of the result, for backward compatibility
+	if not vim.list_contains(fts, lang) then
+		table.insert(fts, lang)
+	end
+	return fts
+end
+
 local function from_cursor_pos()
 	-- get_parser errors if parser not present (no grammar for language).
 	local has_parser, parser = pcall(vim.treesitter.get_parser)
 
 	if has_parser then
 		local cursor = require("luasnip.util.util").get_cursor_0ind()
 		-- assumption: languagetree uses 0-indexed byte-ranges.
-		return {
-			parser
-				:language_for_range({
-					cursor[1],
-					cursor[2],
-					cursor[1],
-					cursor[2],
-				})
-				:lang(),
-		}
+		local lang = parser
+			:language_for_range({
+				cursor[1],
+				cursor[2],
+				cursor[1],
+				cursor[2],
+			})
+			:lang()
+		return fts_from_ts_lang(lang)
 	else
 		return {}
 	end
 end
 
 local function from_filetype()
-	return vim.split(vim.bo.filetype, ".", true)
+	return vim.split(vim.bo.filetype, ".", { plain = true, trimemtpy = false })
 end
 
 -- NOTE: Beware that the resulting filetypes may differ from the ones in `vim.bo.filetype`. (for