feat(stdlib): vim.json.encode(...,{escape_slash:boolean}) #30561

Problem:
vim.json.encode escapes every slash in string values (for example in
file paths), and is not optional. Use-case is for preventing HTML
injections (eg. injecting `</script>` closing tag); in the context of
Nvim this is rarely useful.

Solution:
- Add a `escape_slash` flag to `vim.json.encode`.
- Defaults to `false`. (This is a "breaking" change, but more like
  a bug fix.)

runtime/lua/vim/_meta/json.lua

@@ -35,5 +35,8 @@ function vim.json.decode(str, opts) end
 
 --- Encodes (or "packs") Lua object {obj} as JSON in a Lua string.
 ---@param obj any
+---@param opts? table<string,any> Options table with keys:
+---                                 - escape_slash: (boolean) (default false) When true, escapes `/`
+---                                                           character in JSON strings
 ---@return string
-function vim.json.encode(obj) end
+function vim.json.encode(obj, opts) end