Add support for form indentation on slurp/barf

[julienvincent]

This adds support for automatic indentation correction when slurping and barfing.

The goal of this implementation is to:

1. Provide a visual aid to the user that allows them to confirm they are operating on the correct node, and to know when to stop when performing recursive slurp/barf operations.
2. Be simple to maintain and understand while being as correct as possible
3. Be replaceable with other implementations.
4. Be as performant as possible. There should be no lag or visual jitter

The goal is not to be 100% correct. If a more correct implementation is needed then one can be provided through indent_fn. For example, an implementation using vim.lsp.buf.format could be built if the user doesn't mind sacrificing performance for correctness.

[armed]

Also added zprint support, it is a third party cli tool and it is much faster than LSP approach. To play around with it add this you your config somewhere:

local function zpr(input)
  -- assuming zprint binary name is "zpr" and it is available in PATH
  local result = vim.fn.system("echo '" .. input .. "' | zpr '{:style [:indent-only :respect-nl]}'")
  if result then
    return result:gsub("\r?\n$", "")
  end
  return input
end

local function zprint_indent(buf, start, end_)
  local api = vim.api
  local lines = api.nvim_buf_get_text(buf, start[1], 0, end_[1], end_[2], {})
  local text = table.concat(lines, "\n")
  local result = zpr(text)
  if result then
    local base_indent = string.rep(" ", start[2])
    local formatted_lines = {}
    for line in result:gmatch("[^\r\n]+") do
      table.insert(formatted_lines, base_indent .. line)
    end
    api.nvim_buf_set_text(buf, start[1], 0, end_[1], end_[2], formatted_lines)
  end
end

and then add an option to point to indent function indent_behaviour = zprint_indent

[armed]

Function zprint_indent is currently not part of a plugin, but we can include it as a general indent wrapper which passes input text and handles output text from functions such as zpr above.

[armed]

my last two commits are assuming that boundary symbol only 1 char length, but it is incorrect in case of #{ #( etc, need to handle that properly also, so still WIP

[armed]

reverted last 4 commits, experiments failed :)

[armed]

super close to perfection :)

except this is not indenting:

@(foo|)
@(bar)

[julienvincent]

Good catch. Fixed!