fix(perf): cache this.exclude.shouldInstrument for improved performance

[sapphi-red]

This PR drastically improves performance for large codebases.
Below is the result of total time collecting coverage for my project which uses c8 internally in vitest.

	Before	After	Diff
with all=false	37.056s	7.126s	-29.93s (-80%)
with all=true	33.515s	2.849s	-30.66s (-91%)

This result was measured by adding console.time and console.timeEnd around here.

c8/lib/report.js

Lines 66 to 78 in ae1f5e5

	const context = libReport.createContext({
	dir: this.reportsDirectory,
	watermarks: this.watermarks,
	coverageMap: await this.getCoverageMapFromAllCoverageFiles()
	})
	for (const _reporter of this.reporter) {
	reports.create(_reporter, {
	skipEmpty: false,
	skipFull: this.skipFull,
	maxCols: 100
	}).execute(context)
	}

This is a bit off topic but FYI this this.exclude.globSync(workingDir) call takes 3.6s totally.
So there is a room to improve more if I can change implementation inside test-exclude.

refs: vitest-dev/vitest#1075 (comment)

Checklist

• npm test, tests passing
• tests and/or benchmarks are included

[bcoe]

This seems like a great improvement, thank you.

[bcoe]

@sapphi-red thanks for the great performance improvement.

I think you might need to run the snapshot for Node 10, since it looks like this changes its output slightly.

[sapphi-red]

@bcoe Thank you for reviewing! I have updated snapshot for Node 10.