feat(filetype): vim.filetype.inspect() returns copy of registry

Problem: No way of inspecting the (user-added) filetype detection rules.

Solution: Add `vim.filetype.inspect()` returning copies of the internal
`extension`, `filename`, `pattern` tables. Due to the dynamic nature of
filetype detection, this will in general not allow getting the list of
known filetypes, but at least one can see if a given extension is known.

runtime/doc/lua.txt

@@ -2409,6 +2409,20 @@ vim.filetype.get_option({filetype}, {option})
     Return: ~
         (`string|boolean|integer`) Option value
 
+vim.filetype.inspect()                                *vim.filetype.inspect()*
+    Inspect the current state of the filetype registry.
+
+    Returns a copy of the internal tables used for filetype detection by
+    extension, filename, or pattern. Note: Due to the dynamic nature of
+    filetype detection, this is only useful for checking whether a certain
+    extension, filename, or pattern has been registered so far. In addition,
+    the `pattern` table is in an internal format optimized for fast lookup.
+    Prefer |vim.filetype.match()| for checking the detected filetype for a
+    given pattern.
+
+    Return: ~
+        (`table<string, table<string, vim.filetype.mapping|table<string, vim.filetype.mapping>>>`)
+
 vim.filetype.match({args})                              *vim.filetype.match()*
     Perform filetype detection.
 

runtime/doc/news.txt

@@ -299,6 +299,8 @@ LUA
 • |vim.json.encode()| `sort_keys` option sorts by key.
 • |vim.json.decode()| `skip_comments` option allows comments in JSON data.
 • EXPERIMENTAL: |vim.pos|, |vim.range| provide Position/Range abstraction.
+• |vim.filetype.inspect()| returns a copy of the internal tables used for
+  filetype detection.
 
 OPTIONS
 

runtime/lua/vim/filetype.lua

@@ -3295,4 +3295,20 @@ function M.get_option(filetype, option)
   return require('vim.filetype.options').get_option(filetype, option)
 end
 
+--- Inspect the current state of the filetype registry.
+---
+--- Returns a copy of the internal tables used for filetype detection by extension, filename, or
+--- pattern. Note: Due to the dynamic nature of filetype detection, this is only useful for checking
+--- whether a certain extension, filename, or pattern has been registered so far. In addition, the
+--- `pattern` table is in an internal format optimized for fast lookup. Prefer |vim.filetype.match()|
+--- for checking the detected filetype for a given pattern.
+---@return table<string, table<string, vim.filetype.mapping|table<string, vim.filetype.mapping>>>
+function M.inspect()
+  return {
+    extension = vim.deepcopy(extension),
+    filename = vim.deepcopy(filename),
+    pattern = vim.deepcopy(pattern),
+  }
+end
+
 return M