refactor: use `trim_start_matches` to remove tilde and at symbols

[lavafroth]

Changes

• Replaced the body of the strip_at function to call trim_start_matches on a string to strip the @ and ~ chars.
• Removed obsolete strip_tilde function.
• Cached the results of strip_at applied on token_new into tokens_no_at. The values are used in the subsequent for loop and for building the modifiers vector.

Tests are passing.

[Shinyzenith]

CC: @angelofallars would you mind reviewing this? If not, I'll do the needful.

[Shinyzenith]

CC: @zubairmh

[Shinyzenith]

I approve these changes from my end but Ideally I think we shouldn't improve the hand rolled parser anymore -- the transition to the grammar based solution would be much better. Lets see what zubair thinks 😄