feat(config): Setting default config directory using env NVRS_CONFIG_DIR

[ZennoZenith]

Using environment variable to set default config location #41

Description

Using env NVRS_CONFIG_DIR to set default config director
Precedence order is

• custom location defined via --config
• NVRS_CONFIG_DIR
• XDG_CONFIG_HOME
• ~/.config/nvrs

Context

• Resolves Using environment variable to set default config location #41

Testing

Screenshots / Logs (if applicable)

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I made corresponding changes to the documentation.
• I formatted the code with rustfmt.
• I checked the lints with clippy.

[ZennoZenith]

I haven't updated the docs

[koibtw]

thanks for the PR!

[koibtw]

would also be nice to implement the fix mentioned in #40 (comment) but i'll probably do that in a separate PR

[koibtw]

wouldn't a NVRS_CONFIG_FILE approach be better?

[ZennoZenith]

I used directory because maybe

oldver = "oldver.json"
newver = "newver.json"
keyfile = "n_keyfile.toml"

files could be stored in this directory by default

[koibtw]

would be nice to implement this for keyfile & verfiles.
+ i think it would work better as a CLI argument, not a variable.