[decimal] Implement sqrt for BigDecimal

[forfudan]

This pull request introduces several enhancements to the BigDecimal type in the DeciMojo library, including the addition of square root functionality and related tests. The most important changes include updates to the BigDecimal struct, new exponential functions, and corresponding unit tests.

Enhancements to BigDecimal type:

• src/decimojo/bigdecimal/bigdecimal.mojo: Added sqrt method to compute the square root of a BigDecimal number with a specified precision, and an is_negative method to check if a number is negative. [1] [2]

New exponential functions:

• src/decimojo/bigdecimal/exponential.mojo: Implemented the sqrt function to calculate the square root of a BigDecimal number, handling special cases and performing Newton's method iterations for precision.

Unit tests for exponential functions:

• tests/bigdecimal/test_bigdecimal_exponential.mojo: Added tests for the sqrt function, including cases for perfect squares, non-perfect squares, decimal inputs, large numbers, small numbers, scientific notation, and precision tests.
• tests/bigdecimal/test_data/bigdecimal_exponential.toml: Created test data for the square root tests, covering a variety of input scenarios.

Other updates:

• README.md: Updated the core types table to include aliases for BigUInt, BigInt, Decimal, and BigDecimal.
• benches/bigdecimal/bench.mojo: Added benchmarking support for the square root operation. [1] [2] [3]