CLI rename

[1st1]

We should start renaming the CLI:

• Add support for the new "gel://" schema
• "gel.toml" support. The new CLI should display a warning if "edgedb.toml" is used. It should error out if both "edgedb.toml" and "gel.toml" are present and not identical (I think it's OK if someone wants to commit a symlink to make sure they transition client versions gradually)
• Add support for ".gel" files. We should emit warning for ".esdl" files, but we can support them almost indefinitely.
• the "gel" command should install itself on edgedb cli upgrade, potentially replacing edgedb with a symlink to gel binary
• Help strings need to be updated
• The CLI should support GEL_* environment variables. If there are two same variables GEL_XX and EDGEDB_XX, the former should take precedence and a warning should be emitted.

cc @elprans @mmastrac @msullivan @scotttrinh