Create 64-bit builds

[nosoop]

Introduces 64-bit support for TF2Items.

For those building it, make sure your SourceMod dependencies are up to date (it uses safetyhooks and hl2sdk-manifests).

python ..\configure.py --hl2sdk-root ${HL2SDK_ROOT} --mms-path ${MMSOURCE_112} --sm-path ${SOURCEMOD_112} -s tf2 --hl2sdk-manifest-path ${HL2SDK_MANIFEST_DIR} --targets x86_64

Currently WIP; working on the following:

• [x] Bring back the plugin compile stage
• [ ] Verify Linux build
• [x] Fix packaging script to place 64-bit binaries in the correct location
• [x] Get the Github Actions workflow set up to create releases

[nosoop]

I haven't checked the Linux build nor tested the API extensively but I'll just leave it to someone else to figure out if there's any issues with it.