meson format: removes leading spaces before method calls on their own lines

[dcbaker]

Describe the bug
Mesa (and I suspect other projects that make heavy use of feature option methods, has a lot of code like this):

opt = get_option(...) \
  .enable_if(<some condition is met>, error_message : ...) \
  .enable_auto_if(<some other condition is met>) \
  .disable_if(<some final condition is met>, error_message : ...)

Meson format removes these leading spaces, making the code harder to read.

Expected behavior
I would prefer it to just leave the spaces alone. However, I know a heuristic to handle that might be difficult, so even an option like clang-format's // clang-format off/// clang-format on would be a nice step, and might be generally useful on it's own for other corner cases that meson format doesn't yet handle correctly.

system parameters
Meson 1.5.0 and master (cdf646f)

[annacrombie]

@dcbaker I'm gonna take black's stance on this and say that you should just wrap that whole thing in () so the formatter doesn't have to mess with line continuations. Also, muon fmt supports magic comments to enable / disable formatting, with # fmt: off and #fmt: on1 (in case you wanted to use the same syntax @bruchar1).