Support decimal type in RLC

[MaryGao]

fixes #2157
Support decimal in RLC - map the decimal to number in JS directly, and will

• add below comments in generated code

Please note the field was supposed to be a decimal but JavaScript does not have a native 'BigDecimal' data type.
So it was converted to a number instead. It is recommended to use a third-party library like 'decimal.js' to handle 
any calculations.

• report a warning

Please note the decimal type will be converted to number. If you strongly care about precision you can use @encode to encode it as a string for the property - {your prop name}.

Encoding as string is still pending typespec discussion microsoft/typespec#2762.

[joheredi]

Left a couple of comments but look fine otherwise