Add user-defined literals for string_view

[loliGothicK]

This pull request makes several enhancements and corrections to the unindent library. The key changes include:

• README.md:
  • Updated the documentation to reflect the addition of new user-defined literals (_iv and _i1v).
  • Detailed the behavior of each new literal and provided examples.
  • Added a new section explaining the differences between _i and _iv (_i1 and _i1v) and how they interact with std::format.
  • Updated descriptions and examples to use basic_fixed_string instead of fixed_string.
• include/unindent/unindent.hpp:
  • Renamed fixed_string to basic_fixed_string to better reflect its purpose as a structural type.
  • Added new user-defined literals (_iv and _i1v) and their corresponding implementations.
  • Introduced alias templates and deduction guides for basic_fixed_string.
• tests/test.cpp:
  • Updated test cases to use basic_fixed_string instead of fixed_string.
  • Added new test cases for the _iv and _i1v user-defined literals to ensure correct behavior.

[yukatayu]

読みましたが変な点は見当たらないです。
良さそう…！