Rollup issue for completion (ctrl+space, ctrl+J, etc. - not autocompletion)

Tracking multiple issues here so they don't get lost in the sea of other issues.

Ctrl+space completion (called "completion" from here on out) is plain buggy in a number of cases:

- [ ] Invoking in the middle of a symbol doesn't preselect a useful entry: #2200
- [x] Completing when an underscore has been entered appends the whole token, not the remaining pieces: #3406
- [x] Completing when a number has been entered appends the whole token, not the remaining pieces: #4515
- [x] IntelliSense list is filtered by the characters _after_ a number or underscore, not the whole token: #6134