Handle casing properly in word prediction

[enaboapps]

Tracks the casing plan for word prediction suggestions and phrases.

• Send the real Shift state to the prediction worker #907 Send the real Shift state to the prediction worker
• Keep the model's capital on proper nouns mid-sentence #908 Keep the model's capital on proper nouns mid-sentence
• Capitalise I and its contractions in suggestions #909 Capitalise I and its contractions in suggestions
• Make one sentence-start decision shared by keyboard and worker #910 Make one sentence-start decision shared by keyboard and worker
• Continue an all-caps typed prefix in uppercase #911 Continue an all-caps typed prefix in uppercase
• Retype the typed prefix when its case differs from the accepted suggestion #912 Retype the typed prefix when accepting restores a capital in it
• Casing matrix tests and documentation for word prediction #913 Casing matrix tests and documentation

Items 1 to 3 are independent. Item 4 touches both processes. Item 5 needs its rule confirmed. Item 6 is a product decision.

[enaboapps]

All seven items are merged: #915, #916, #917, #918, #919, #920 and #921. Casing rules, their limits and a table of outcomes are in docs/word-prediction.md, and the casing_matrix test covers every combination.