Add PWM speaker output for RP2040 and RP2350

[deadprogram]

Adds a `pwm` package that plays audio on a speaker or passive piezo buzzer through a PWM pin, as an alternative to an I2S DAC. It implements `tinyplayer.Output` so the player needs no changes.

• Mono, about 11 bits, with the PWM carrier at 40 kHz or above. Lower sample rates repeat each sample to keep the carrier inaudible.
• DMA writes the duty values, paced by the PWM wrap DREQ, so there is no CPU work per sample.
• An optional second pin on the same slice gets the inverted signal, for driving a piezo across two pins.
• Build the examples with `-tags speaker` to use GPIO2 and GPIO3.
• README covers wiring for a piezo, a transistor driven speaker and an amplifier, and notes that active buzzers only play tones.
• Adds a Japanese speech example sound made with VOICEVOX.

Tested on a xiao-rp2350 with a DFRobot 386AMP module and speaker on D8. All example sounds play at the right speed at 8, 16 and 44.1 kHz. Unit tests cover the sample conversion and timing, and the smoketest builds the speaker example for pico and xiao-rp2350.