…5469)

Most LSPs will complete case-insensitive matches, particularly from
lowercase to uppercase.  In some cases, notably Pyright, this is given
as a simple insert text instead of TextEdit.  When this happens, the
prefix text was left unedited.

This commit adds new functions to `Transaction` that allow creating
edits that might potentially overlap. Any change that overlaps
previous changes is ignored. Furthermore, a utility method is added
that also drops selections associated with dropped changes (for
transactions that are created from a selection).

This is needed to avoid crashes when applying multicursor
autocompletions, as the edit from a previous cursor may overlap
with the next cursor/edit.

Multicursor completions may overlap and therefore overlapping
completions must be dropped to avoid crashes. Furthermore, multicursor
edits might simply be out of range if the word before/after the cursor
is shorter. This currently leads to crashes, instead these selections
are now also removed for completions.

This commit also significantly refactors snippet transaction generation
so that tabstops behave correctly with the above rules. Furthermore,
snippet tabstops need to be carefully mapped to ensure their position
is correct and consistent with our selection semantics. Finally,
we now keep a partially updated Rope while creating snippet
transactions so that we can fill information into snippets that
depends on the position in the document.