fix(init): skip existing files instead of erroring (#20434)

### What
Skip writing files from the template if the files already exist in the
project directory.

### Why
When I run deno init in a directory that already has a main.ts, or one
of the other template files, I usually want to initialize a workspace
around a file I've started working in. A hard error in this case seems
counter productive. An informational message about what's being skipped
seems sufficient.

Close #20433

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