feat: add per-language template file support

Problem: new solution files were always created empty, requiring users
to manually paste boilerplate or rely on editor snippets that fire
outside cp.nvim's control.

Solution: add an optional template field to the language config. When
set to a file path, its contents are written into every newly created
solution buffer before the setup_code hook runs. Existing files are
never overwritten.

lua/cp/config.lua

@@ -7,6 +7,7 @@
 ---@class CpLanguage
 ---@field extension string
 ---@field commands CpLangCommands
+---@field template? string
 
 ---@class CpPlatformOverrides
 ---@field extension? string
@@ -215,6 +216,10 @@ local function validate_language(id, lang)
     commands = { lang.commands, { 'table' } },
   })
 
+  if lang.template ~= nil then
+    vim.validate({ template = { lang.template, 'string' } })
+  end
+
   if not lang.commands.run then
     error(('[cp.nvim] languages.%s.commands.run is required'):format(id))
   end