This slightly degrades the performance of CJS export analysis on
subsequent runs because I changed it to no longer cache in the DENO_DIR
with this PR (denort now properly has no idea about the DENO_DIR). We'll
have to change it to embed this data in the binary and that will also
allow us to get rid of swc in denort (will do that in a follow-up PR).
This commit adds "deno add" subcommand that has a basic support for
adding "jsr:" packages to "deno.json" file.
This currently doesn't support "npm:" specifiers and specifying version
constraints.