Freeze src in initialize

[jhawthorn]

This freezes @src inside of initialize, which allows for Erb.new(...).freeze to be Ractor-shareable (previously Ractor.make_shareable which applies recursively would have been necessary).

One explicit freeze of erb is still necessary because of filename=/lineno=/location=

[G-Rath]

fwiw I've found that versions of tilt before v2.5.0 break as a result of this - I don't think it's a big deal even though technically you could say this is a breaking change, but I wanted to post it somewhere in case someone has an older app like we did that was not on the latest version :)

Our erb got bumped indirectly when updating action_text-trix to fix a security issue