feat: add ESlint rule no-sparse-arrays for config check

[KristjanESPERANTO]

Adding a rule to the config checker config so that unexpected commas in the middle of arrays (reported in issue #3910) are better detected.

Two commas in a row inside the modules array create an empty entry (undefined). JavaScript accepts that syntax, but MagicMirror would later try to load that “module” and fail.

Alternatively, we could filter out undefined entries, but with this PR, the user receives a clear message indicating where the error lies, can easily fix it, and thus has a cleaner configuration.

Before

[2025-10-10 19:33:30.874] [INFO]  Checking config file /home/kristjan/MagicMirror/config/config.js ... 
[2025-10-10 19:33:30.944] [INFO]  Your configuration file doesn't contain syntax errors :) 
[2025-10-10 19:33:30.945] [INFO]  Checking modules structure configuration ... 
[2025-10-10 19:33:31.027] [ERROR] This module configuration contains errors:
undefinedmust be object

After

[2025-10-10 19:41:20.030] [INFO]  Checking config file /home/kristjan/MagicMirror/config/config.js ... 
[2025-10-10 19:41:20.107] [ERROR] Your configuration file contains syntax errors :(
Line 91 column 1: Unexpected comma in middle of array.