Implement SFX & BGM Data API Functions: get_lengths

[RobDavenport]

Now that bgm_is_active and channel_is_active are removed, we need a way to notify the game logic how long a sound will last.

I propose adding two more functions to the data_api

1. bgm_length_secs(index: i32) -> f32 which returns the length of the BGM in seconds.
2. bgm_length_frames(index: i32) -> i32 which returns the length of the BGM in frames.
3. sfx_length_secs(index: i32) -> f32 same as 1, but for SFX.
4. sfx_length_frames(index: i32) -> i32 same as 2, but for SFX.

While we don't have looping BGM or SFX yet, I figure we could use INFINITY or NaN or a negative number to represent looping ones in the future.

For the "frame" based ones, this should be an integer for the sound to completely complete. For example, if a SFX would take 15.5 game frames to complete, it should return 16.

Task List:

• Figure out how to calculate length of a sequence in seconds (f32) from the BPM
• Figure out how to calculate the length of a sequence in frames, from the previous step.
• implement bgm_length_secs
• implement bgm_length_frames
• implement sfx_length_secs
• implement sfx_length_frames
• update gamercade_rs with 4 new functions
• update docs with 4 new functions (gamercade-io/gamercade_site@c1675e7)
• Add UI label on song editor to show song length
• Add UI label on SFX editor to show sfx length