Added Z-function implementation

[CarolLuca]

I added an implementation of the Z-function of a string (also known as the Z-algorithm).

[codecov[bot]]

Codecov Report

Merging #523 (55a7ae2) into main (1aa53bf) will increase coverage by 0.011%. The diff coverage is 100.000%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #523 +/- ## ============================================= + Coverage 98.546% 98.558% +0.011% ============================================= Files 32 32 Lines 4059 4092 +33 ============================================= + Hits 4000 4033 +33 Misses 59 59 ``` | [Impacted Files](https://codecov.io/gh/codezonediitj/pydatastructs/pull/523?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj) | Coverage Δ | | |---|---|---| | [pydatastructs/strings/algorithms.py](https://codecov.io/gh/codezonediitj/pydatastructs/pull/523?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj#diff-cHlkYXRhc3RydWN0cy9zdHJpbmdzL2FsZ29yaXRobXMucHk=) | `100.000% <100.000%> (ø)` | | [](https://codecov.io/gh/codezonediitj/pydatastructs/pull/523?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj)

[czgdp1807]

See, https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/523/blob/pydatastructs/strings/algorithms.py. L206 and L231 are not tested. When query = "", do we even need to do any kind of processing. In KMP we just do, return DynamicOneDimensionalArray(int, 0) if, text or query is empty. Why can't we do that in your implementation?

[CarolLuca]

See, app.codecov.io/gh/codezonediitj/pydatastructs/pull/523/blob/pydatastructs/strings/algorithms.py. L206 and L231 are not tested. When query = "", do we even need to do any kind of processing. In KMP we just do, return DynamicOneDimensionalArray(int, 0) if, text or query is empty. Why can't we do that in your implementation?

I see that the case is treated. The _z_vector() function doesn't do this because the actual vector it is often used also without a query, but for the _z_function(), I see I implemented the case when the lengths of the strings are zero. If I'm wrong please correct me, I might have missed something.