Add support for case-insensitive regex

[fsasm]

In some languages identifiers and keywords are case-insensitive. Some examples are Ada, VHDL and SQL. Also literals in other languages like C and C++ can be in parts case-insensitive.

For these cases it would be helpful to mark regexes to match case-insensitive. In flex regexes that are case-insensitive are prefixed with ?i:.