Add copy construction support

Explain that each thread should have its own parser object.

README.md

@@ -53,7 +53,7 @@ Note: for current users of *TinyExpr++*, please see the [compatibility advisory]
 - Supports C and C++ style comments within math expressions.
 - Released under the zlib license - free for nearly any use.
 - Easy to use and integrate with your code.
-- Thread-safe; parser is in a self-contained object.
+- Thread-safe; parser can be constructed as per thread objects.
 
 ## Changes from TinyExpr
 

docs/manual/index.qmd

@@ -46,4 +46,4 @@ It's open-source, free, easy-to-use, and self-contained in a single source and h
 - Can be configured to use `double`, `long double`, or [`float`](#te-float) for its calculations.
 - Released under the zlib license - free for nearly any use.
 - Easy to use and integrate with your code.
-- Thread-safe; parser is in a self-contained object.
+- Thread-safe; parser can be constructed as per thread objects.

tinyexpr.h

@@ -246,9 +246,23 @@ class te_parser
     /// @private
     te_parser() = default;
     /// @private
-    te_parser(const te_parser&) = delete;
+    te_parser(const te_parser& that)
+        {
+        m_customFuncsAndVars = that.m_customFuncsAndVars;
+        m_unknownSymbolResolve = that.m_unknownSymbolResolve;
+        m_keepResolvedVarialbes = that.m_keepResolvedVarialbes;
+        m_decimalSeparator = that.m_decimalSeparator;
+        m_listSeparator = that.m_listSeparator;
+        }
     /// @private
-    te_parser& operator=(const te_parser&) = delete;
+    te_parser& operator=(const te_parser& that)
+        {
+        m_customFuncsAndVars = that.m_customFuncsAndVars;
+        m_unknownSymbolResolve = that.m_unknownSymbolResolve;
+        m_keepResolvedVarialbes = that.m_keepResolvedVarialbes;
+        m_decimalSeparator = that.m_decimalSeparator;
+        m_listSeparator = that.m_listSeparator;
+        }
 
     /// @private
     ~te_parser() { te_free(m_compiledExpression); }