Make source_location() and SourceFile's and line_starts consistent when the last source byte is a newline

[mlechu]

When constructing SourceFile's line_starts, we currently pretend there's a
newline at the end of the source if there isn't one, and then ignore the last
newline unconditionally. This became a problem in LSP testing, where calling
source_location with a cursor position at the end of the file would give a
wrong result only if the last character was a newline.

julia> source_location(SourceFile("\n\n\n"), 1)
(1, 1)

julia> source_location(SourceFile("\n\n\n"), 2)
(2, 1)

julia> source_location(SourceFile("\n\n\n"), 3)
(3, 1)

julia> source_location(SourceFile("\n\n\n"), 4)
(3, 2)

This change removes the conditional line and the code stripping it. This also
brings line_starts closer to its definition (one entry per line in the file).

Alternatively, we could add the dummy line unconditionally. It would make for
calculating the length of the last real line easier, but would make
line_starts less explainable.

[aviatesk]

This behavior feels more natural for what source_location returns, and it also seems like the implementation gets a bit simpler as a result.

[aviatesk]

@c42f I'm hoping to move this PR forward and release a new patch version soon. Could you give us some comments on this if you have any?