feat: Support control characters in surrounds. (#179) (#211)

BREAKING CHANGE: User defined `invalid_key_behavior` handlers will be
activated for control characters that don't have defined `surrounds`.

Supports Vim's notation of special characters in `surrounds`. One can simply write:

```lua
surrounds = {
    ["<BS>"] = { ... }
},
```

lua/nvim-surround/config.lua

@@ -227,14 +227,23 @@ M.default_opts = {
         },
         invalid_key_behavior = {
             add = function(char)
+                if char:find("%c") then
+                    return
+                end
                 return { { char }, { char } }
             end,
             find = function(char)
+                if char:find("%c") then
+                    return
+                end
                 return M.get_selection({
                     pattern = vim.pesc(char) .. ".-" .. vim.pesc(char),
                 })
             end,
             delete = function(char)
+                if char:find("%c") then
+                    return
+                end
                 return M.get_selections({
                     char = char,
                     pattern = "^(.)().-(.)()$",
@@ -583,6 +592,8 @@ M.translate_opts = function(user_opts)
                 "nvim-surround"
             )
         end
+        -- Support Vim's notation for special characters
+        char = vim.api.nvim_replace_termcodes(char, true, true, true)
         -- Check if the delimiter has not been disabled
         if not user_surround then
             opts.surrounds[char] = false

lua/nvim-surround/input.lua

@@ -4,12 +4,11 @@ local M = {}
 ---@return string? @The input character, or nil if a control character is pressed.
 ---@nodiscard
 M.get_char = function()
-    local ret_val, char_num = pcall(vim.fn.getchar)
-    -- Return nil if error (e.g. <C-c>) or for control characters
-    if not ret_val or type(char_num) ~= "number" or char_num < 32 then
+    local ret_val, char = pcall(vim.fn.getcharstr)
+    -- Return nil if error (e.g. <C-c>) or for <Esc>/<C-[> (ascii code 27)
+    if not ret_val or char == "\27" then
         return nil
     end
-    local char = vim.fn.nr2char(char_num)
     return char
 end