Suggest stringr as needed by create_project() (#47)

Previously this soft dependency doesn't need to be explicitly declared because **rmarkdown** has been suggested, which requires **stringr** (which in turn requires **stringi**). Now the dependency is no longer guaranteed in **rmarkdown** (rstudio/rmarkdown#2530), so **starter** has to explicitly declare the dependency, otherwise it will fail `R CMD check`.

DESCRIPTION

@@ -35,6 +35,7 @@ Suggests:
     pkgdown (>= 1.6.1),
     readr (>= 2.0.1),
     rmarkdown (>= 2.11),
+    stringr,
     spelling (>= 2.2),
     testthat (>= 3.0.4)
 VignetteBuilder: