feat(ui2): lift ui2 options into 'messagesopt' (part 1) #41474

Problem:
In order for ui2 to graduate to the main "messages ui" its configuration
needs to graduate into actual options.

Solution:
Migrate some of its config to 'messagesopt':
- "maxheight" (note: currently this is an integer treated as
  a "percentage"; if we want to support a row count we could allow
  values with units, like `"42%"`)
- "pager"
- "timeout"

Also improves error messages:

    messagesopt=hit-enter,history:500,bogus       E474: Unknown item 'bogus'
    messagesopt=hit-enter,history:500,progress:x  E474: 'progress' must be one of: , c
    messagesopt=hit-enter,history:abc             E474: 'history' requires a number

0 files changed, 0 insertions, 0 deletions