2.6.2 - Critical bugfix to "burn bootloader", and Wire fixes

2.6.1 contained a bug that prevented "burn bootloader" from working correctly. This is corrected.

This version also contains a further-improved version of Wire, as usual, thanks to the hard work of @MX682X who did a considerable amount of work to ensure that sleep functioned correctly on I2C slave devices. including testing on an astounding scale to verify the absence of cursed intervals during which an attempt to talk to the device would instead lead to it dragging down the bus until a reset or power cycle (most people who have worked with I2C have met at ke. There are a few new hacky ugly bits too (namely, the compile time errors for clearly wrong values passed to pins() and swap() are back - they were lost with the wire rewrite a while back, and the there is now an option to configure the hodge podge of strange compatibility options that the TWI of modern AVRs has. This is mostly directed towards the Dx-series, where the SMBus levels enable communication with newfangled sensors with 1.8-2v logic levels even without using MVIO - this version of Wire is identical to what will be in the next version of DxCore, but this also lets you configure hold times if you need to work with something that is designed such that it requires meeting SMBus specs), and when some part is being uncooperative, gives you some the two timing-related knobs to turn in the hopes of fixing it).

Half duplex serial should now work correctly.

The documentation has been improved.

Warnings associated with the Event library on some parts are corrected.

Some newly added or altered functionality was not getting proper keyword highlighting.