Add separate "continuation indent space" setting

"tab_space = 2" will set my block indent to 2, but it also sets the indentation for continued lines (function params, long lines, etc) to 2 as well. It would be great to have a separate option for continued lines.

For example, the Google Style guide uses and indent=2 and continued_indent=4, which for me is more readable as I can easily see which indentations are blocks and which are continued lines just by scanning left alignment.

Perhaps we can add a 'continued_tab_space' option? To avoid changing behavior for just setting "tab_space = 2", perhaps we can default continued_tab_space to be unset, and when it is unset defer to tab_space for both?

Start by locating how the tab_space setting controls block and continued-line indentation in rustfmt. Add a separate continued_tab_space setting with the requested fallback to tab_space when unset, then verify both the default behavior and distinct Google-style values through the formatter's existing tests.