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`.

In 3a4a66017b74192caaf9af9af172bdc08e0c1608, 4d3a67cd620152d11ab9b5f5bdd973f84cc2d44b
we renamed "buffer" to "buf" in dict parameters.

This commit also renames such keys in dict return-values.

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.

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: nvim_exec_autocmds() documentation incorrectly describes the default
for "pattern" as *, when it's actually the current file name (like :doautocmd).

Solution: correct it. Add a test.

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.