Update strings.jl to add British_National_Grid

[TimG1964]

Added "British_National_Grid" => EPSG{27700} to esriid2code dict.

[juliohm]

Thank you @TimG1964 💯

Notice that we need additional steps to fully address #131. But we can move step by step so that you learn more about it.

Will merge and release a patch after tests are completed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.35%. Comparing base (2ed3a59) to head (5b143a4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #132   +/-   ##
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  Coverage   95.35%   95.35%           
=======================================
  Files          27       27           
  Lines        1205     1205           
=======================================
  Hits         1149     1149           
  Misses         56       56           

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