Support variable substitution

[SqlQuantumLeap]

Allow substitution of $(var_name) variable.

This also requires handling:

• -v command-line switch
• :setvar SQLCMD command
• checking all 3 levels of environment variables
• -x ignore scripting variables

The following notes are taken from the MSDN page for the sqlcmd Utility:

Variable Precedence (Low to High)

1. System-level environmental variables.
2. User-level environmental variables
3. Command shell (SET X=Y) set at command prompt before running sqlcmd.
4. sqlcmd -v X=Y
5. :Setvar X Y

NOTES:

• SQLCMD processes commands and variable substitution per each batch!
• Scripting variable names are case-insensitive
• If no value is assigned to a sqlcmd environment variable, the variable is removed.
• Using :setvar VarName without a value clears the variable.
• SQLCMD commands inside of block comments and string literals are not processed.
• See also: Use sqlcmd with Scripting Variables