make diagnostics stick to word boundaries

Diagnostics are currently extended if text is inserted at their end. This is
desirable when inserting text after an identifier. For example consider:

let foo = 2;
    --- unused variable

Renaming the identifier should extend the diagnostic:

let foobar = 2;
    ------ unused variable

This is currently implemented in helix but as a consequence adding whitespaces
or a type hint also extends the diagnostic:

let foo      = 2;
    -------- unused variable
let foo: Bar = 2;
    -------- unused variable

In these cases the diagnostic should remain unchanged:

let foo      = 2;
    --- unused variable
let foo: Bar = 2;
    --- unused variable

As a heuristic helix will now only extend diagnostics that end on a word char
if new chars are appended to the word (so not for punctuation/ whitespace).
The idea for this mapping was inspired for the word level tracking vscode uses
for many positions. While VSCode doesn't currently update diagnostics after
receiving publishDiagnostic it does use this system for inlay hints for example.
Similarly, the new association mechanism implemented here can be used for word
level tracking of inlay hints.

A similar mapping function is implemented for word starts. Together
these can be used to make a diagnostic stick to a word. If that word
is removed that diagnostic is automatically removed too. This is the exact
same behavior VSCode inlay hints eixibit.

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