Is your feature request related to a problem? Please describe.

nix-shell will use default.nix or shell.nix when default.nix doesn't exist.
nix-shell /a/path/of/a/directory will use /a/path/of/a/directory/default.nix

Describe the solution you'd like
When /a/path/of/a/directory/default.nix doesn't exist, search /a/path/of/a/directory/shell.nix.

Additional context
currently, nix-shell /a/path/of/a/directory/shell.nix can work.

Priorities

Low.

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