flash: add -o flag support to save built binary (Fixes #4937)

[rdon-key]

This PR adds -o flag support to the tinygo flash command, allowing users to save the built binary to a specified output file. This brings flash in line with the behavior of tinygo build, promoting consistency and improving usability.

Fixes #4937

Changes

• Added -o flag support to the flash command
• Introduced validateOutputFormat function to check compatibility with the target format
• Saved output file after build and before flashing
• Updated help text for the flash command with usage examples

Behavior

• When -o is specified, the binary is both saved and flashed
• Output file format must match the target's expected format (e.g., .uf2, .hex)
• If the format is invalid, the command fails before building
• The saved file is identical to what is flashed

Testing Results

# Build works as usual
$ tinygo build -o build.uf2 --target waveshare-rp2040-zero .
   code    data     bss |   flash     ram
 258388    1664    5352 |  260052    7016

# Flash with -o saves file and flashes the device
$ tinygo flash -o flash.uf2 --target waveshare-rp2040-zero .
   code    data     bss |   flash     ram
 258388    1664    5352 |  260052    7016

# Format validation works
$ tinygo flash -o flash.hex --target waveshare-rp2040-zero .
output format .hex does not match target format .uf2

# File content is identical between build and flash
$ sha256sum flash.uf2 build.uf2
95cc493412d9937433cd0ba9e8d667be92de19bab18eaba6cc49ada4b8d3a8c5 *flash.uf2
95cc493412d9937433cd0ba9e8d667be92de19bab18eaba6cc49ada4b8d3a8c5 *build.uf2

# Updated Help Output
$ tinygo help flash
Flash the program to a microcontroller. Some common flags are described below.

        -target={name}:
                        Specifies the type of microcontroller that is used. The name of the
                        microcontroller is given on the individual pages for each board type
                        listed under Microcontrollers
                        (https://tinygo.org/docs/reference/microcontrollers/).
                        Examples: "arduino-nano", "d1mini", "xiao".

        -o={filename}:
                        Save the built binary to the specified output file. The file
                        format must match the target's expected format (e.g., .hex,
                        .uf2). Both flashing and saving will be performed.

        -monitor:
                        Start the serial monitor (see below) immediately after
                        flashing. However, some microcontrollers need a split second
                        or two to configure the serial port after flashing, and
                        using the "-monitor" flag can fail because the serial
                        monitor starts too quickly. In that case, use the "tinygo
                        monitor" command explicitly.

[b0ch3nski]

Your code checks for this condition already before calling this function, so this seems redundant