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.

src/nvim/options.lua

@@ -6627,6 +6627,9 @@ return {
         top are deleted if new lines exceed this limit.
         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.
       ]=],
       full_name = 'scrollback',
       redraw = { 'current_buffer' },