Add get symbol public member function and 2.1.0 version setup

[martin-olivier]

Description

• Public member function dylib::get_symbol has been added
• Public member type definition dylib::native_symbol_type has been added
• Documentation has been improved
• dylib 2.1.0 version setup

Changes include

• [ ] Bugfix (non-breaking change that solves an issue)
• [X] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (change that is not backwards-compatible and/or changes current functionality)

[codecov[bot]]

Codecov Report

Merging #56 (e62c61a) into v2.X.X (ccb26ab) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            v2.X.X       #56   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           59        59           
  Branches         3         3           
=========================================
  Hits            59        59           

Impacted Files	Coverage Δ
include/dylib.hpp	100.00% <100.00%> (ø)

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