Update to Swift 5.9

[fpseverino]

These changes are now available in 1.12.0

• Update swift-tools-version to 5.9
• Add common swiftSettings to targets
• Fix unhandled files warnings
  • Templates folder in test target; added to target's resources
  • Docs.docc folder in main target; updating swift-tools-version fixes it
• Lock was deprecated in favour of NIOLock
• Add MUSL and iOS in CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.34%. Comparing base (f6a08a1) to head (20faf17).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   82.09%   82.34%   +0.24%     
==========================================
  Files          25       25              
  Lines        2603     2617      +14     
==========================================
+ Hits         2137     2155      +18     
+ Misses        466      462       -4     

Files with missing lines	Coverage Δ
Sources/LeafKit/LeafCache/DefaultLeafCache.swift	75.67% <ø> (ø)
Sources/LeafKit/LeafCache/LeafCache.swift	0.00% <ø> (ø)
Sources/LeafKit/LeafData/LeafData.swift	60.00% <100.00%> (ø)
Sources/LeafKit/LeafRenderer.swift	94.01% <100.00%> (ø)
Sources/LeafKit/LeafSerialize/LeafSerializer.swift	95.20% <100.00%> (ø)
...rces/LeafKit/LeafSerialize/ParameterResolver.swift	71.21% <ø> (ø)
Sources/LeafKit/LeafSource/LeafSources.swift	95.74% <100.00%> (+2.12%)	⬆️
Sources/LeafKit/LeafSource/NIOLeafFiles.swift	89.13% <100.00%> (-8.70%)	⬇️
Sources/LeafKit/LeafSyntax/LeafSyntax.swift	90.65% <100.00%> (ø)
Sources/LeafKit/LeafSyntax/LeafTag.swift	77.77% <ø> (-0.80%)	⬇️

... and 2 files with indirect coverage changes

[fpseverino]

The Docs.docc problem is solved with any version higher than 5.4, I put 5.8 only because vapor/leaf is on 5.8

[gwynne]

The correct minimum to use at this point in time is 5.9.

[gwynne]

Ideally you would also enable various Swift settings (most especially strict concurrency), but that would likely entail a good bit of work to clear up Sendable issues.

[fpseverino]

Ok, I have added the common 5.9 Swift settings, the only one that required immediate changes is "ExistentialAny", strict concurrency just gives a bunch of warnings.
Should we keep the setting and warnings or should I disable it for now?

[fpseverino]

This PR could be revisited

[0xTim]

Keep the warnings for now (they shouldn't appear in adopters packages I don't think) and then we can follow up with #125