Add a direnv health check (flake-specific)

[srid]

This check should be disabled by default, and only enabled when nix-health.default.direnv.enable = true; is in the flake (per #60).

It should check:

• direnv is in PATH, else point to https://zero-to-flakes.com/direnv/
• direnv status contains Found RC allowed true, else ask the user to run direnv allow

[shivaraj-bh]

For now, I can parse direnv status and check for the line Found RC allowed true to verify if direnv is allowed, in future parsing it from json would be better. ref: direnv/direnv#244 (comment)

[srid]

I think this check should be disabled by default, and only enabled when nix-health.default.direnv.enable = true; is in the flake (per #60).

It should check:

• direnv is in PATH, else point to https://zero-to-flakes.com/direnv/
• direnv status contains Found RC allowed true, else ask the user to run direnv allow

This is enough to use this check in nammayatri.

I'll update the description accordingly.

[srid]

I'm gonna move this to nix-browser repo.

[srid]

juspay/omnix#70