Problem:
- Not obvious which _meta/ are generated and which should be edited
  manually.
- The require guard (`error('Cannot require a meta file')`) is not
  consistently present in all meta files.

Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.

Problem: Cannot set highlight group fonts via API, only via :highlight
command.

Solution: Add font parameter in nvim_set_hl().

Problem: hlgroup2dict passes &ns_id to ns_get_hl twice. The first call
(link=true) sets *ns_hl = 0 when link_global is set, so the second call
and the sg_cleared guard both see ns_id == 0 and bail out. The group is
silently dropped from the result.

Solution: use a temporary copy of ns_id for each ns_get_hl call so the
original value is preserved.

For now, add a private "_truncate" flag to nvim_echo, using a truncation
method similar to showmode().

Problem:
`:help dev-name-common` states that "buf" should be used instead of
"buffer" but there are cases where buffer is mentioned in the lua API.

Solution:
- Rename occurrences of "buffer" to "buf" for consistency with the
  documentation.
- Support (but deprecate) "buffer" for backwards compatibility.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

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.

Problem: nvim_set_hl always replaces all attributes.

Solution: Add update field. When true, merge with existing
attributes instead of replacing. Unspecified attributes are preserved.
If highlight group doesn't exist, falls back to reset mode.

Problem: nvim_open_tabpage's "enter" argument is optional, which is inconsistent
with nvim_open_win.

Solution: make it a (non-optional) positional argument, like nvim_open_win.

Also change "enter"'s description to be more like nvim_open_win's doc.

Problem: no API function for opening a new tab page and returning its handle, or
to open without entering.

Solution: add nvim_open_tabpage.

Problem: nvim_win_get_config() does not return a window's "style".

Solution: always include it, and document `style=""`.

Always included so it can be used reciprocally with nvim_open_win() or
nvim_win_set_config(). (otherwise the config of a window with kWinStyleUnused
will not unset the kWinStyleMinimal style of another window if passed to
nvim_win_set_config, for example)