feat(term): increase max scrollback to 1000000

Problem:
Cannot use `nvim_open_term()` to pipe terminal scrollback > 100000

Solution:
Increase scrollback limit to 1000000

If there's no technical consequences of doing this, can be set even
higher in the future.

runtime/doc/news.txt

@@ -300,6 +300,7 @@ These existing features changed their behavior.
 • 'spellfile' location defaults to `stdpath("data").."/site/spell/"` instead of
   the first writable directory in 'runtimepath'.
 • |vim.version.range()| doesn't exclude `to` if it is equal to `from`.
+• 'scrollback' maximum value increased from 100000 to 1000000
 
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*

runtime/doc/options.txt

@@ -5151,7 +5151,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 			local to buffer
 	Maximum number of lines kept beyond the visible screen. Lines at the
 	top are deleted if new lines exceed this limit.
-	Minimum is 1, maximum is 100000.
+	Minimum is 1, maximum is 1000000.
 	Only in |terminal| buffers.
 
 	Note: Lines that are not visible and kept in scrollback are not

runtime/lua/vim/_meta/options.lua

@@ -5395,7 +5395,7 @@ vim.wo.scr = vim.wo.scroll
 
 --- Maximum number of lines kept beyond the visible screen. Lines at the
 --- top are deleted if new lines exceed this limit.
---- Minimum is 1, maximum is 100000.
+--- Minimum is 1, maximum is 1000000.
 --- Only in `terminal` buffers.
 ---
 --- Note: Lines that are not visible and kept in scrollback are not