Move Sail Isla plugin into main repository

[Alasdair]

No changes in the PR other than silencing a few warnings by adding error cases for missing patterns.

Should fix issues with this plugin breaking by making sure it is in the main Sail CI and releases.

[github-actions]

Test Results

   16 files     36 suites   0s ⏱️
1 015 tests 1 012 ✅  3 💤 0 ❌
4 911 runs  4 869 ✅ 42 💤 0 ❌

Results for commit 3ff687a.

[pmundkur]

This is great! I'm now able to get what looks like a proper ir that I can run through isla-axiomatic barring a couple of warnings:

Warning: Could not find register __isla_always_aligned when parsing registers.defaults in configuration
Warning: Could not find register __isla_always_aligned when parsing registers.defaults in configuration
No primop parse_hex_bits (Name { id: 2483 })
No primop emulator_write_tag (Name { id: 2781 })
No primop emulator_read_tag (Name { id: 2780 })
No primop valid_reservation (Name { id: 4078 })
[...]