Relates to #3621.

Initial draft of keyboard to test start-of-sentence detection.

Relates to #3621.

This adds basic compiler support for the `begin newContext` and
`begin postKeystroke` statements, along with the corresponding `gn` and
`gpk` entry points in the compiled JavaScript keyboard.

TODO: `readonly` semantics, usage constraints, `&layerChanged`.

Relates to #3621.

Adds basic support for `begin newContext`.

Relates to #3621.

This adds support for `begin postKeystroke` to KeymanWeb. This is now
close to final, apart from support for `&layerChanged`.

Relates to #3621.

Add support for `&layerChanged` system store. This store is set to `1`
before a `begin postKeystroke`, if the keystroke it follows resulted in
a layer change, either programatically through a keyboard rule, or
through a `nextlayer` property of the touched key.

Relates to #3621.

Adds `readonly` group support and verifies that keyboards meet
constraints around usage of these groups.

(Note: fixes test_valid.kps and test.bat which seemed to have some
invalid tests?)

Picked up in development of #5963.

…-overflow-1

fix(developer): kmdecomp string overflow

Co-authored-by: Darcy Wong <darcy_wong@sil.org>

Relates to #3621.

Adds `&layoutChanged`, `begin newcontext`, `begin postkeystroke` support
to IDE editor and keyword help.

…-ide-support

feat(developer): support for start-of-sentence in IDE 🍆