feat(cli): update the graph arg behavior (#1353)
Updates the behavior of the `--graph` CLI flag and fixes a few bugs.
> This command will generate an svg, png, jpg, pdf, json, html, or [other supported output formats](https://graphviz.org/doc/info/output.html) of the current task graph.
The output file format defaults to jpg, but can be controlled by specifying the filename's extension.
> If Graphviz is not installed, or no filename is provided, this command prints the dot graph to `stdout`
This PR also:
1. Updates docs to reflect the current state of the `--graph` CLI flag
1. Refactors the graph visualization code out of `run.go`
1. Cleans up the file name of colors_cache (follow up from https://github.com/vercel/turborepo/pull/1346)
Fixes https://github.com/vercel/turborepo/issues/1286
