refactor: use nvim_{get,set}_option_value for vim.{b,w}o

`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.

test/functional/lua/vim_spec.lua

@@ -1396,7 +1396,7 @@ describe('lua stdlib', function()
     ]]
     eq('', funcs.luaeval "vim.bo.filetype")
     eq(true, funcs.luaeval "vim.bo[BUF].modifiable")
-    matches("Invalid option name: 'nosuchopt'$",
+    matches("unknown option 'nosuchopt'$",
        pcall_err(exec_lua, 'return vim.bo.nosuchopt'))
     matches("Expected lua string$",
        pcall_err(exec_lua, 'return vim.bo[0][0].autoread'))
@@ -1415,7 +1415,7 @@ describe('lua stdlib', function()
     eq(0, funcs.luaeval "vim.wo.cole")
     eq(0, funcs.luaeval "vim.wo[0].cole")
     eq(0, funcs.luaeval "vim.wo[1001].cole")
-    matches("Invalid option name: 'notanopt'$",
+    matches("unknown option 'notanopt'$",
        pcall_err(exec_lua, 'return vim.wo.notanopt'))
     matches("Expected lua string$",
        pcall_err(exec_lua, 'return vim.wo[0][0].list'))