Fix {{#if_pure}} syntax issue where indentation and whitespace were not preserved as intended.

[Bo26fhmC5M]

PR Checklist

• Did you check if it works normally in all models? ignore this when it dosen't uses models
• Did you check if it works normally in all of web, local and node hosted versions? if it dosen't, did you blocked it in those versions?
• Did you added a type def?

Description

This PR fixes an issue where indentation and whitespace inside the {{#if_pure}} syntax were being trimmed.
I made an effort to minimize the impact on the parser's existing behavior.