Enhancement: fields in ToggleTermConfig with default value should be marked as optional

[lungsin]

Currently, when a config is annotated with ---@type ToggleTermConfig, the lsp displays warning that a lot of fields are missing:

Missing required fields in type `ToggleTermConfig`: `shade_filetypes`, , ....., `responsiveness` Lua Diagnostics. (missing-fields) [9, 10]