fix(snippets): provide textEdit field for builtin snippet

[soifou]

Completion items didn't include the textEdit field before, so blink.cmp had to guess where to insert them. That guess was sometimes off, e.g., when the character before the cursor is the same as the first character of the expanded snippet, leading to incorrect insertion.

Closes #2159

[saghen]

We should include the whole keyword in the text edit range, which you can grab from context.bounds. I haven't tested, but I'd guess that if you manually <C-space> at test|, accepting the snippet would delete only the final t

[saghen]

Thinking about this more, we should set the text edit to the guessed range based on only the label. Otherwise, snippets with label ;foo will only replace the foo on execution.

The issue is https://github.com/saghen/blink.cmp/blob/main/lua/blink/cmp/lib/text_edits.lua#L201 calls into https://github.com/saghen/blink.cmp/blob/main/lua/blink/cmp/fuzzy/rust/lib.rs#L166, which gets the edit range from all of label, filterText and insertText. The insertText contains the expanded snippet and iirc there were some issues with lazydev.nvim around this (ref). Might have my facts wrong here though, it's been a while...

Maybe we could skip guessing on insertText for snippets generally, but I think I prefer guessing the range specifically for this source and setting the textEdit.