Support omitting exception variable in bs files and convert exception var to an Expression

[TwitchBronBron]

Our current CatchStatement stores the exception variable as a Token. This PR converts the CatchStatement.exceptionVariable property to an Expression, which supports VariableExpression and TypeCastExpression. This should more closely align with how TypeScript supports catch(e as <any type you want here>). This was a breaking change to the AST, so it needed to be done as part of v1.0.0

Also supports completely omitting the exception variable when you don't need to use it.

function init()
    try
        somethingDangerous()
    catch
        print "I live on the edge"
    end try
end function

transpiles to

function init()
    try
        somethingDangerous()
    catch e ' or __bsc_error if there's a local variable conflict
        print "I live on the edge"
    end try
end function