feat(term): enable reflow by default (#21124)

Problem: Contents of terminal buffer are not reflown when Nvim is
resized.

Solution: Enable reflow in libvterm by default. Now that libvterm is
vendored, also fix "TUI rapid resize" test failures there.

Note: Neovim's scrollback buffer does not support reflow (yet), so lines
vanishing into the buffer due to a too small window will be restored
without reflow.

runtime/lua/vim/_meta/options.lua

@@ -5253,6 +5253,9 @@ vim.wo.scr = vim.wo.scroll
 --- Minimum is 1, maximum is 100000.
 --- Only in `terminal` buffers.
 ---
+--- Note: Lines that are not visible and kept in scrollback are not
+--- reflown when the terminal buffer is resized horizontally.
+---
 --- @type integer
 vim.o.scrollback = -1
 vim.o.scbk = vim.o.scrollback