fix: add ESLint 10 compatibility by replacing deprecated context.getSourceCode()

[aklkv]

Summary

Adds ESLint 10 compatibility by replacing the deprecated context.getSourceCode() API with the backward-compatible pattern context.sourceCode ?? context.getSourceCode().

Fixes #2378

Changes

• Updated 60 occurrences of context.getSourceCode() across 35 files to use the backward-compatible pattern
• Added CI job to test against ESLint 8, 9, and 10

Details

ESLint 10 removed the deprecated context.getSourceCode() method. The new pattern works with ESLint 8, 9, and 10:

// Before
const sourceCode = context.getSourceCode();

// After
const sourceCode = context.sourceCode ?? context.getSourceCode();

[NullVoxPopuli]

The eslint matrix failed. Tyty for doing this!

[aklkv]

I will circle back to it once typescript-eslint/typescript-eslint#11952 is resolved as it doesn't even seem to be feasible to upgrade just yet