Added variable prefix option in inspector_gadget.gd

[Ev01]

This option will only show variables with the specified prefix at the start of the variable name.
For example, say you have a file that looks like this and only want the variables that start with "exposed_" to be seen in the inspector:

# Example File
export (Array) exposed_array_var = []
export (Int) exposed_int_var = 5
export (int) var_that_should_not_be_seen = 0

Setting the prefix option to "exposed_" will make only the exposed variables shown in the inspector, and will remove the prefix from the display name. In the inspector, they will be seen as Array Var and Int Var.
This is useful for differentiating variables that should be private and variables that should be exposed in code. It's different from using blacklist because it removes the prefix from the display name and is also easier to set up with scripts that have a lot of variables.