fix: method and action IR names are now separated by `__` instead of `_`

[mhasel]

Updates the IR names of methods and actions to adhere to the scheme <type-name>__<method-name>.

Also adds additional FB_INIT tests to ensure proper stack-variable initialization.

Can you add a similar test in codegen? I would like to see the case where we have a function using a function block, and that functionblock has a user_init be tested. I expect the function, while initializing the function block call to (implicitly) call fb_init. I'm fine with adding this as a followup if we know the case works, but it would be good to express this in codegen tests

Originally posted by @ghaith in https://github.com/PLC-lang/rusty/pull/1458#discussion_r2062875975

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.11%. Comparing base (e731987) to head (1b087f7). Report is 66 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1471      +/-   ##
==========================================
+ Coverage   93.67%   94.11%   +0.43%     
==========================================
  Files         166      173       +7     
  Lines       50097    56526    +6429     
==========================================
+ Hits        46928    53199    +6271     
- Misses       3169     3327     +158     

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