feat: improve setup and new command

[dsherret]

Running deno run -A jsr:@deno/wasmbuild new now gets a project all the way to a
working state instead of leaving the deno.json setup to the reader.

On top of scaffolding the rs_lib crate, the new command now:

1. Adds a wasmbuild task to the deno.json file—the same task the manual setup
   instructions describe ("wasmbuild": "deno run -A @deno/wasmbuild"), creating
   the file when it doesn't exist and filling in only what's missing when it does.
2. Runs deno install jsr:@deno/wasmbuild in the folder.

The config file is edited with @david/jsonc-morph,
so comments, key order, and existing formatting all survive—a deno.jsonc file
(or a deno.json with comments, which Deno also accepts) gets filled in like any
other config. A deno.json takes precedence over a deno.jsonc when both exist,
matching Deno's own resolution, and an existing wasmbuild task is never
overwritten.

The README is restructured to lead with the scaffold command and keep the manual
setup as the alternative.

[dsherret]

The problem with having a default --minimum-depenency-age=P1D is that this command will not work for most people for 1 day. Not a big deal though.

[dsherret]

Overall, this is better, but also it's more necessary because Deno had a breaking change to discover the local lockfile when using global commands. This led to many CI failures for me (I do like it better though).