Allow breaking after = in named arguments when name = value exceeds line-width

[randy3k]

Closes #527.

Previously, fmt_argument_fields() always formatted named arguments with an unconditional space() between name = and value:

write!(f, [name_clause.format(), space(), value.format()])

When a named argument has a long name (e.g., backticked stopifnot(`...` = ...) messages) or an expression where name = value exceeds line-width (80 columns), keeping value glued to name = either forces the line past line-width or forces value itself to break across multiple lines even when value would fit flat on the next line.

This PR updates fmt_argument_fields() to:

1. Preserve space() after = (no break) for:
   • Arguments inside test blocks (test_that() / describe()) to keep test descriptions on the opening line.
   • Braced expressions ({ ... }), function definitions (function(...), \(...)), and control flow statements (if, for, while, repeat).
   • Function calls whose callee name alone exceeds line-width.
2. For other expressions, wrap the line break after = in a dedicated group (group(&indent(&soft_line_break_or_space()))) and conditionally indent the value (indent_if_group_breaks(&format_value, group_id)).
3. To prevent expressions from prematurely breaking internally before breaking after =, flatten soft line breaks in format_value (or the <left> <op> prefix for multi-line binary expressions) when the span fits on a single indented line.

Examples

Before

# 1. String literal or atomic value where `name = value` exceeds 80 columns
foo(
  x,
  long_argument_name = "a_very_long_string_value_that_exceeds_eighty_columns_when_on_one_line",
  `allow_null is not supported; please provide a non-null input value` = !allow_null
)

# 2. Named arguments where `name = value` exceeds 80 columns on one line
stopifnot(
  `Provide equal-length x and y vectors, or a single y value` = lengths_match(
    x,
    y
  ) ||
    length(y) == 1L,
  "wait_done should be a logical scalar" = is_scalar_logical(wait_done) &&
    !is.na(wait_done),
  `file_path should be a single file name or an open connection` = is_character_or_connection(
    file_path
  )
)

mutate(
  df,
  branch_exit_fallback_calls = c(
    "warning",
    "warn",
    "message",
    "cat",
    "stopifnot"
  )
)

# 3. Pipe chains where `name = <lhs> |>` exceeds 80 columns
process_data(
  data = starwars |>
    filter(species == "Human") |>
    select(name, height, mass),
  filtered_human_character_subset_from_starwars_dataset = starwars_character_data |>
    filter(species == "Human") |>
    select(name, height, mass)
)

After

# 1. String literal or atomic value breaks cleanly after `=`
foo(
  x,
  long_argument_name =
    "a_very_long_string_value_that_exceeds_eighty_columns_when_on_one_line",
  `allow_null is not supported; please provide a non-null input value` =
    !allow_null
)

# 2. Breaking after `=` keeps `value` flat on a single indented line when it fits
stopifnot(
  `Provide equal-length x and y vectors, or a single y value` =
    lengths_match(x, y) || length(y) == 1L,
  "wait_done should be a logical scalar" =
    is_scalar_logical(wait_done) && !is.na(wait_done),
  `file_path should be a single file name or an open connection` =
    is_character_or_connection(file_path)
)

mutate(
  df,
  branch_exit_fallback_calls =
    c("warning", "warn", "message", "cat", "stopifnot")
)

# 3. Pipe chains break after `=` only when `name = <lhs> |>` exceeds 80 columns
process_data(
  data = starwars |>
    filter(species == "Human") |>
    select(name, height, mass),
  filtered_human_character_subset_from_starwars_dataset =
    starwars_character_data |>
      filter(species == "Human") |>
      select(name, height, mass)
)

Unchanged (preserved on the = line)

# Braced expressions, functions, and control flow stay on the `=` line
foo(
  long_argument_name_that_never_breaks_before_braces = {
    x + 1
  },
  long_argument_name_that_never_breaks_before_anon_fn = function(a, b) a + b,
  long_argument_name_that_never_breaks_before_lambda = \(a, b) a + b
)

# Short argument names with multi-line calls expand their own arguments without breaking after `=`
mutate(
  name = if_else(
    str_sub(name, 1, 3) %in% c("AL0", "AL1"),
    name,
    str_to_title(name)
  )
)