ICU-22261 Add tech preview implementation for MessageFormat 2.0 to i…

…cu4c

Adds an implementation of the draft MessageFormat 2.0 spec, found at
https://github.com/unicode-org/message-format-wg

icu4c/source/common/unicode/utypes.h

@@ -438,6 +438,7 @@ typedef enum UErrorCode {
 
     U_PLUGIN_CHANGED_LEVEL_WARNING = -120, /**< A plugin caused a level change. May not be an error, but later plugins may not load. */
 
+
 #ifndef U_HIDE_DEPRECATED_API
     /**
      * One more than the highest normal UErrorCode warning value.
@@ -568,12 +569,24 @@ typedef enum UErrorCode {
     U_FORMAT_INEXACT_ERROR,           /**< Cannot format a number exactly and rounding mode is ROUND_UNNECESSARY @stable ICU 4.8 */
     U_NUMBER_ARG_OUTOFBOUNDS_ERROR,   /**< The argument to a NumberFormatter helper method was out of bounds; the bounds are usually 0 to 999. @stable ICU 61 */
     U_NUMBER_SKELETON_SYNTAX_ERROR,   /**< The number skeleton passed to C++ NumberFormatter or C UNumberFormatter was invalid or contained a syntax error. @stable ICU 62 */
+
+    /* MessageFormat 2.0 errors */
+    U_UNRESOLVED_VARIABLE_ERROR,    /** A variable is referred to but not bound by any definition */
+    U_SYNTAX_ERROR,                 /** Includes all syntax errors */
+    U_UNKNOWN_FUNCTION_ERROR,       /** An annotation refers to a function not defined by the standard or custom function registry */
+    U_VARIANT_KEY_MISMATCH_ERROR,   /** In a match-construct, one or more variants had a different number of keys from the number of selectors */
+    U_FORMATTING_ERROR,             /** Covers all runtime errors: for example, an internally inconsistent set of options. */
+    U_NONEXHAUSTIVE_PATTERN_ERROR,  /** In a match-construct, the variants do not cover all possible values */
+    U_DUPLICATE_OPTION_NAME_ERROR,  /** In an annotation, the same option name appears more than once */
+    U_SELECTOR_ERROR,               /** A selector function is applied to an operand of the wrong type */
+    U_MISSING_SELECTOR_ANNOTATION_ERROR,  /** A selector expression evaluates to an unannotated operand */
+
 #ifndef U_HIDE_DEPRECATED_API
     /**
      * One more than the highest normal formatting API error code.
      * @deprecated ICU 58 The numeric value may change over time, see ICU ticket #12420.
      */
-    U_FMT_PARSE_ERROR_LIMIT = 0x10114,
+    U_FMT_PARSE_ERROR_LIMIT = 0x10125,
 #endif  // U_HIDE_DEPRECATED_API
 
     /*

icu4c/source/common/utypes.cpp

@@ -129,6 +129,15 @@ _uFmtErrorName[U_FMT_PARSE_ERROR_LIMIT - U_FMT_PARSE_ERROR_START] = {
     "U_FORMAT_INEXACT_ERROR",
     "U_NUMBER_ARG_OUTOFBOUNDS_ERROR",
     "U_NUMBER_SKELETON_SYNTAX_ERROR",
+    "U_UNRESOLVED_VARIABLE_ERROR",
+    "U_SYNTAX_ERROR",
+    "U_UNKNOWN_FUNCTION_ERROR",
+    "U_VARIANT_KEY_MISMATCH_ERROR",
+    "U_FORMATTING_ERROR",
+    "U_NONEXHAUSTIVE_PATTERN_ERROR",
+    "U_DUPLICATE_OPTION_NAME_ERROR",
+    "U_SELECTOR_ERROR",
+    "U_MISSING_SELECTOR_ANNOTATION_ERROR"
 };
 
 static const char * const