Prior to this commit, only the letters 'A' - 'Z' (ignoring case) were
supported in identifiers (i.e., property, field, and variable names).
This known (yet undocumented) limitation prevented the use of characters
such as 'ü', 'ñ', 'é' as well as letters from other character sets such
as Chinese, Japanese, Cyrillic, etc.
This commit lifts that restriction by delegating to Character.isLetter()
to determine if a character in a SpEL expression is a letter.
Closes gh-30580
c8c8f5722b