`cpp_source(file = )` default to active file

[romainfrancois]

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Related to https://github.com/rstudio/rstudio/issues/10337 it would be useful if the default value of cpp_source(file=) was the active file, or if we had a cpp_source_active_file().

This can take inspiration from devtools::test_active_file(): https://github.com/r-lib/devtools/blob/a999c84c03a8c955bf1bc81b2ae3f6d8e1c7110b/R/test.R#L59