* refactor(lsp): use autocmd api

* refactor(lsp): inline BufWritePost and VimLeavePre callbacks

This fixes the following bugs:
`${1:else_text}` -> format with if_text: "else_text"
`${1:-else_text}` -> format with if_text: "else_text"
`${1:}` in `format` (eg. empty else_text) -> error.
`${1:}` (eg. empty placeholder) -> error.

Thanks hrsh7th :)

Some language servers send empty `textDocument/publishDiagnostics`
messages after indexing the project with URIs corresponding to unopened buffers.
This commit guards against opening buffers corresponding to empty diagnostics.

This change forces the start of an incremental sync range to begin always on an existing line.

Closes #16624

Fixes two issues with aligning the start position and end position to
codepoints when calculating the start and end range.

When aligning the start position:
* use aligned byte index to calculate character index rather than 
  the unadjusted byte

When aligning the end position:
* do not adjust the end byte if it falls on a UTF-8 codepoint
* align byte to the first byte of the next codepoint rather than the
  last byte of the current codepoint
* compute character character end range on the aligned byte index

This commit also adds additional test coverage, including multibyte operations
that previously failed before this commit.

closes https://github.com/neovim/neovim/issues/16352

* improve handling of multi-byte deletions

* use codeunits/points instead of byte ranges when applicable
* take into account different file formats when computing range and
  sending text (dos, unix, and mac supported)
* add tests of incremental sync