refactor(diagnostic): remove get_virt_text_chunks()

This function isn't compatible with including diagnostic sources when
"source" is "if_many" since it only has access to diagnostics for a
single line. Rather than having an inconsistent or incomplete interface,
make this function private. It is still exported as part of the module
for backward compatibility with vim.lsp.diagnostics, but it can
eventually be made into a local function.

runtime/doc/deprecated.txt

@@ -75,7 +75,9 @@ For each of the functions below, use the corresponding function in
 *vim.lsp.diagnostic.get_prev()*
 *vim.lsp.diagnostic.get_prev_pos()*
 *vim.lsp.diagnostic.get_virtual_text_chunks_for_line()*
-			Use |vim.diagnostic.get_virt_text_chunks()| instead.
+				No replacement. Use options provided by
+				|vim.diagnostic.config()| to customize
+				virtual text.
 *vim.lsp.diagnostic.goto_next()*
 *vim.lsp.diagnostic.goto_prev()*
 *vim.lsp.diagnostic.redraw()*		Use |vim.diagnostic.show()| instead.

runtime/doc/diagnostic.txt

@@ -333,26 +333,6 @@ get_prev_pos({opts})                           *vim.diagnostic.get_prev_pos()*
                 Return: ~
                     table Previous diagnostic position as a (row, col) tuple.
 
-                                       *vim.diagnostic.get_virt_text_chunks()*
-get_virt_text_chunks({line_diags}, {opts})
-                Get virtual text chunks to display using
-                |nvim_buf_set_extmark()|.
-
-                Parameters: ~
-                    {line_diags}  table The diagnostics associated with the
-                                  line.
-                    {opts}        table|nil Configuration table with the
-                                  following keys:
-                                  • prefix: (string) Prefix to display before
-                                    virtual text on line.
-                                  • spacing: (number) Number of spaces to
-                                    insert before virtual text.
-
-                Return: ~
-                    array of ({text}, {hl_group}) tuples. This can be passed
-                    directly to the {virt_text} option of
-                    |nvim_buf_set_extmark()|.
-
 goto_next({opts})                                 *vim.diagnostic.goto_next()*
                 Move to the next diagnostic.