Change es2015 usages to existing ts functions

[j-oliveras]

Fixes #28918 and complements PR #28921.

Changes:

• Already reported on Avoid uses of ES6 Set, use Array instead #28921, String.prototype.startsWith instead of ts.startsWith: src/compiler/inspectValue.ts line 151.

• Two usages of String.prototype.endsWith instead of ts.endsWith: src/services/rename.ts line 48.

• Array.prototype.find instead of ts.find: src/compiler/moduleSpecifiers.ts line 206.

[DanielRosenwasser]

Thanks!