Problem:
LSP spec uses the term "position encoding" where we say "offset encoding".

Solution:
- Rename it everywhere except `vim.lsp.Client.offset_encoding` (which would be breaking).
- Mention "position encoding" in the documentation for `vim.lsp.Client.offset_encoding`.

* deprecate old signatures
* move to new str_byteindex/str_utfindex signature
* use single-underscore name (double-underscore is reserved for Lua itself)

Problem:
Some language servers (e.g., rust-analyzer, texlab) are desynced when
the user deletes the entire contents of the buffer. This is due to the
discrepancy between how nvim computes diff and how nvim treats empty
buffer.
* diff: If the buffer became empty, then the diff includes the last
  line's eol.
* empty buffer: Even if the buffer is empty, nvim regards it as having
  a single empty line with eol.

Solution:
Add special case for diff computation when the buffer becomes empty so
that it does not include the eol of the last line.

Problem:

The documentation flow (`gen_vimdoc.py`) has several issues:
- it's not very versatile
- depends on doxygen
- doesn't work well with Lua code as it requires an awkward filter script to convert it into pseudo-C.
- The intermediate XML files and filters makes it too much like a rube goldberg machine.

Solution:

Re-implement the flow using Lua, LPEG and treesitter.

- `gen_vimdoc.py` is now replaced with `gen_vimdoc.lua` and replicates a portion of the logic.
- `lua2dox.lua` is gone!
- No more XML files.
- Doxygen is now longer used and instead we now use:
  - LPEG for comment parsing (see `scripts/luacats_grammar.lua` and `scripts/cdoc_grammar.lua`).
  - LPEG for C parsing (see `scripts/cdoc_parser.lua`)
  - Lua patterns for Lua parsing (see `scripts/luacats_parser.lua`).
  - Treesitter for Markdown parsing (see `scripts/text_utils.lua`).
- The generated `runtime/doc/*.mpack` files have been removed.
   - `scripts/gen_eval_files.lua` now instead uses `scripts/cdoc_parser.lua` directly.
- Text wrapping is implemented in `scripts/text_utils.lua` and appears to produce more consistent results (the main contributer to the diff of this change).

* refactor(lsp): move changetracking to separate file

- Prefixed changetracking types with `vim.lsp.`

* fixup!: make _reset_timer a local function

* fixup!: remove @private annotations

* fixup!: changetracking.lua -> _changetracking.lua

* fixup! types

* fixup! add send_changes_for_group

* docs(lua): teach lua2dox how to table

* docs(lua): teach gen_vimdoc.py about local functions

No more need to mark local functions with @private

* docs(lua): mention @nodoc and @meta in dev-lua-doc

* fixup!

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

---------

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