Issue Type: Bug

When editing a new GitHub issue from within VSCode, I'll get lots of spurious insertions of user names as I type, meaning I'm often cleaning up after this extension. Here is an example:

Notice how I'm just typing normally, no @ or <tab>, but a random name comes out of nowhere. In case helpful, I did it again, this time slowing down long enough for the pop-up to appear:

I believe the issue is that the suggestions are quite lenient, and hitting <space> accepts the completion. A couple solutions I could think of:

• Only make suggestions if the token starts with @ (or if not desired to have this be default behaviour, at least adding a setting where I can force this)
• Have a setting to disable <space> to accept suggestion

Extension version: 0.34.3
VS Code version: Code 1.63.2 (899d46d82c4c95423fb7e10e68eba52050e30ba3, 2021-12-15T09:37:28.172Z)
OS version: Darwin x64 21.2.0
Restricted Mode: No

vsliv368:30146709
vsreu685:30147344
python383:30185418
vspor879:30202332
vspor708:30202333
vspor363:30204092
pythontb:30283811
pythonptprofiler:30281270
vshan820:30294714
vstes263:30335439
vscorecescf:30384386
pythondataviewer:30285071
vscod805:30301674
pythonvspyt200:30340761
binariesv615:30325510
bridge0708:30335490
bridge0723:30353136
vsaa593:30376534
pythonvs932:30410667
vscop804cf:30404767
vscop453:30404998
vsrem710cf:30416617
vsccppwtstudy:30421978