Add vector normalization method

[mixern6]

The implementation is taken from hl2sdk-csgo.

Vector::Normalized is present in:

• hl2sdk-blade
• hl2sdk-bms
• hl2sdk-csgo
• hl2sdk-doi
• hl2sdk-insurgency
• hl2sdk-pvkii
• hl2sdk-sdk2013

But missing in:

• hl2sdk-css
• hl2sdk-dods
• hl2sdk-episode1
• hl2sdk-hl2dm
• hl2sdk-l4d
• hl2sdk-l4d2
• hl2sdk-nucleardawn
• hl2sdk-orangebox
• hl2sdk-tf2

Please, add it to all the branches where it is not present.