Update for TIP-551:

Add documentation for this feature to the expr man page.  The keyword
"integer value" has been added to the string and expr man page.

Added TCL_PARSE_NO_UNDERSCORE flag so that the digit separator can be
disabled when need when calling TclParseNumber.

Disabled digit separator in the "scan" command when scanning integers
and floating-point numbers.  This is the one place where existing code
may rely on number parsing to stop at an underscore.

Disallow underscore between the leading 0 and the radix specifiers
'x', 'o', 'b', and 'd'.

Added tests for disallowed underscore use and scan with underscores
between digits in the source string.