fix: update theme stylesheet access pattern

[danielbogomazov]

Fixes #2336

This PR replaces bracket notation for theme access with optional chaining to align with the Theme interface style.

Motivation

The current implementation has a discrepancy between the Theme interface defined in TypeScript versus the JavaScript styling logic. This can lead to a poor developer experience, as the interface should serve as a guide for interacting with the library. The following examples illustrate the need for this change.

Example 1: Conforms to the Theme Interface but Does Not Apply the Custom Styling

In this example, the code adheres to the Theme interface, but the JavaScript does not apply the theme's styling correctly.

import { Calendar } from 'react-native-calendars';
import { Theme } from 'react-native-calendars/src/types';

export default function Foo() {
  const theme: Theme = {
    stylesheet: {
      calendar: {
        header: {
          monthText: {
            color: 'red',
          },
        },
      },
    },
  };

  return <Calendar theme={theme} />;
}

Example 2: Applies Custom Styles but Does Not Conform to the Theme Interface

In this case, custom styles are applied, but the implementation does not conform to the Theme interface, leading to TypeScript complaints.

import { Calendar } from 'react-native-calendars';
import { Theme } from 'react-native-calendars/src/types';

export default function Foo() {
  const theme: Theme = {
    'stylesheet.calendar.header': {
      monthText: {
        color: 'red',
      },
    },
  };

  return <Calendar theme={theme} />;
}