[UART] Add buffer level check for synchronous API

[cujomalainey]

Is your feature request related to a problem? Please describe.
Most common uart APIs have the ability to either peek or check the buffer level without actually reading the first element.

• Arduino's available()
• Mbed's readable()
• ESP-IDF's uart_get_buffered_data_len()

The current Zephyr UART driver does not offer this ability.
The only ways to tell if there is data in the buffer is the ISR callback event
or
uart_irq_rx_ready which is only allowed inside the ISR.

Describe the solution you'd like
A method of checking the amount of frames available outside the ISR. Either new syscall or make uart_irq_rx_ready safe outside the ISR.

Describe alternatives you've considered
I can write my own buffer backed by the ISR, but it seems impractical for my use case and seems like this is a common feature that should be available.

Additional context
I can submit the PR, I just wanted to get feedback before doing the work to see why this was not part of the original API.