feat(progress): set Progress-event pattern to "source" #38495

Problem:
Currently, there's no way to distinguish progress messages coming from
different sources. Nor can Progress event be easily filtered based on
source.

Solution:
- Add "source" field to nvim_echo-opts.
- The Progress event pattern is now defined by the "source" field.
- Include the "title" as ev.data.
- Unrelated change: set force=false to disable nesting.

runtime/lua/vim/_meta/api_keysets.lua

@@ -242,6 +242,7 @@ error('Cannot require a meta file')
 --- @field title? string
 --- @field status? string
 --- @field percent? integer
+--- @field source? string
 --- @field data? table<string,any>
 
 --- @class vim.api.keyset.empty