feat(treesitter): use weak tables when possible

Also add the defaulttable function to create a table whose values are created when a key is missing.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

fix(api): nvim_set_option_value for global-local options

global-local window options need to be handled specially. When `win` is
given but `scope` is not, then we want to set the local version of the
option but not the global one, therefore we need to force
`scope='local'`.

Note this does not apply to window-local only options (e.g. 'number')

Example:

   nvim_set_option_value('scrolloff', 10, {})       -- global-local window option; set global value
   nvim_set_option_value('scrolloff', 20, {win=0})  -- global-local window option; set local value
   nvim_set_option_value('number', true, {})        -- local window option

is now equivalent to:

   nvim_set_option_value('scrolloff', 10, {})
   nvim_set_option_value('scrolloff', 20, {win=0, scope='local'})  -- changed from before
   nvim_set_option_value('number', true, {win=0})                  -- unchanged from before

Only the global-local option with a `win` provided gets forced to local
scope.

`nvim_get_option_value` and `nvim_set_option_value` better handle
unsetting local options. For instance, this is currently not possible:

    vim.bo.tagfunc = nil

This does not work because 'tagfunc' is marked as "local to buffer" and
does not have a fallback global option. However, using :setlocal *does*
work as expected

    :setlocal tagfunc=

`nvim_set_option_value` behaves more like :set and :setlocal (by
design), so using these as the underlying API functions beneath vim.bo
and vim.wo makes those two tables act more like :setlocal. Note that
vim.o *already* uses `nvim_set_option_value` under the hood, so that
vim.o behaves like :set.