fix: improve typing for scoped variables

[stevearc]

Problem:

Seeing diagnostic warnings about undefined field when accessing window/buffer/tab scoped variables

vim.w.my_window_var   -- Undefined field `my_window_var`.

Solution:

More correct type definition for vim.b/w/t that accounts for the raw (non-numeric) variable access. This uses the @field annotation syntax to describe all keys of a certain type (detailed here)

[solson]

+1, I just ran into this when using vim.b.term_title and came to check for a bug report.