[decimal] Re-implement BigDecimal.sqrt() with help of BigUInt.sqrt()

[forfudan]

This pull request includes significant updates to the BigDecimal benchmarking and implementation code. Key changes involve a re-implementation of the sqrt function for improved efficiency and accuracy.

Codebase Enhancements:

• Re-implemented the sqrt function in src/decimojo/bigdecimal/exponential. Mojo:
  • Replaced the old sqrt implementation with a more efficient method using BigUInt.sqrt(), which adjusts the scale and coefficient directly for better performance and accuracy.
  • Introduced a new sqrt_decimal_approach function to preserve the old implementation for reference.

Benchmarking Updates:

• Enhanced the square root benchmark (benches/bigdecimal/bench_bigdecimal_sqrt.mojo):

  • Introduced a PRECISION alias for consistent precision handling across operations.
  • Updated the square root computation to use the new PRECISION value instead of a fixed precision of 28.
  • Added extensive test cases for very large decimal numbers, ranging from 100-word to 5000-word decimals, to stress-test the square root implementation.

• Removed the scale_up_by_power_of_10 benchmark entirely, including its import in benches/bigdecimal/bench.mojo and its invocation in the main function.