Problem:
With the addition of the `:help al` text object, you can now easily
format the whole buffer with `gqal`. However, `vim.lsp.formatexpr` only
uses `textDocument/rangeFormatting` which some language servers (like
gopls) don't support.
Solution:
- Fall back to `textDocument/formatting` if the whole buffer is being formatted
and the server doesn't support `textDocument/rangeFormatting`.
- In theory, these two methods should return the same response if the whole
buffer is being formatted, but I preserved the existing behaviour of
prioritising `textDocument/rangeFormatting` in case that does not hold (i.e.
LS bug).
- Also: `vim.lsp.formatexpr` had no tests at all, so actually add tests for it.
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