use char base startswith and endswith

[SimonCropp]

has better perf

[filzrev]

These modifications seems to corresponding to following Code Analyzer Rules

• CA1865/CA1866

I thought it might be better to change these rules category from suggestion to warning at .editorconfig file.
By these changes. It can enforce rules for future code modifications.
And it can automatically refactoring code by executing dotnet format analyzers command.

Example settings for .editorconfig

#----------------------------------------------
# CodeAnalyzer Rules
#----------------------------------------------
[*.{cs}]

# Performance rules
dotnet_diagnostic.CA1865.severity = warning # CA1865: Use 'string.Method(char)' instead of 'string.Method(string)' for string with single char
dotnet_diagnostic.CA1866.severity = warning # CA1866: Use 'string.Method(char)' instead of 'string.Method(string)' for string with single char