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

libvterm v0.3 supports reflow of terminal buffer when Nvim is resized
Since v0.3 is now a required dependency, enable it by default to find
(and fix) possible issues.

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/doc/news.txt

@@ -58,6 +58,10 @@ The following new APIs or features were added.
 
 • Added inline virtual text support to |nvim_buf_set_extmark()|.
 
+• The terminal buffer now supports reflow (wrapped lines adapt when the buffer
+  is resized horizontally). Note: Lines that are not visible and kept in
+  |'scrollback'| are not reflown.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 

runtime/doc/options.txt

@@ -4913,6 +4913,9 @@ A jump table for the options with a short description can be found at |Q_op|.
 	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.
+
 			*'scrollbind'* *'scb'* *'noscrollbind'* *'noscb'*
 'scrollbind' 'scb'	boolean  (default off)
 			local to window