feat: add shift method to Series

[mvanhorn]

Summary

Adds Series.shift(periods, options) matching the pandas Series.shift() API. Requested in #617.

The method shifts values by N positions and fills the gap with NaN (or a custom fillValue).

const sf = new Series([1, 2, 3, 4, 5]);
sf.shift(1).values;   // [NaN, 1, 2, 3, 4]
sf.shift(-1).values;  // [2, 3, 4, 5, NaN]
sf.shift(2, { fillValue: 0 }).values;  // [0, 0, 1, 2, 3]

Changes

• src/danfojs-base/core/series.ts: new shift() method after tail(), follows the same pattern (jsdoc, copy semantics, index preservation)
• src/danfojs-node/test/core/series.test.ts: 5 test cases covering shift down, shift up, zero shift, custom fill, and overflow

Fixes #617

This contribution was developed with AI assistance (Codex).