refactor: update month0 variable name to month

[aj3sh]

We have used month0 as a variable to indicate that the first month starts at 0. However, there has been inconsistency in its usage. In some places, we use month with 0 as the first month, in others, we use month0, and elsewhere, month with 1 as the first month.

Let's refactor the entire codebase to use month consistently, where the first month starts at 0 (e.g., Baishakh and January). We also need to update all variable names and references where both month and month0 have been used inconsistently, including JSDoc comments where month0 has been mentioned.

/**
* dateConverter.ts
* ...
* Note: There are two types of month variables used in this file.
* The first is `month0`, which represents month values starting from 0,
* for example, 0 for January and 0 for Baishakh.
* The second is `month`, which represents month values starting from 1.
*/