Fake players in scoreboards can't contain # anymore

Partner Rank: 2

Verification builds: 1.13.0.15
Regression builds: 1.12

Summary: If a fake player name in /scoreboard contains a number sign (#), the command fails to run as a syntax error.

Impact: This breaks existing content on the store, e.g. Adventures in Khundalan

Repro Steps:
Run the following commands:

/scoreboard objectives add test dummy
/scoreboard players set #testing test 5

Expected Results: Score for fake player #testing is set to 5.

Observed Results: Syntax error, unexpected "#".

Screenshots/Videos attached: No