Fix/showsigpreview (#1042)

* ensure l:bufferlines is defined when using preview window

previously, when using the preview window to show signatures, the code
failed with an stacktrace like:

    "s:on_stdout client request on_notification() error",
    "Vim(return):E121: Undefined variable: l:bufferlines",
    "function <SNR>118_out_cb[2]..
    <SNR>107_on_stdout[79]..
    <SNR>92_request_on_notification[3]..
    <SNR>93_createNext[1]..
    <SNR>93_subscribeSourceCallback[2]..
    <lambda>738[1]..
    <SNR>94_handle_signature_help[48]..
    lsp#ui#vim#output#preview[39]..
    lsp#ui#vim#output#get_size_info, line 22"

This commit ensures that the required variable is properly defined in
all the situations (popup, nvim, preview window)

* removed superfluous return statement

add goto definition integration tests for same file (#972)

* add goto definition integration tests for same file

Fix textEdit check (#914)

Support any completion-engine (#672)

Co-authored-by: Prabir Shrestha <mail@prabir.me>

Update README.md