Perf: Pre-merge matching files patterns

[matwilko]

In the same vein as #99, I think there's some up-front consolidation of config elements that could be done during normalization when adjacent elements share one or more files patterns in common (taking into account ignores as well).

(Adjacency might not even be required if it can be determined easily enough that two elements are completely disjoint (e.g. in the easiest case of ['**/*.a'] and ['**/*.b']), but I think that's something to leave for later.)

Given that configs are likely to have multiple elements that share some files patterns in common (due to importing configs from plugins that are applied to the same sets of files, e.g. many typescript related plugins will share { files: ['**/*.ts'] } in common), the potential for merging these configs together ahead of time presents the same benefits as merging through the global configurations - fewer elements in the array means less work per-file we get config for.

Even a partial match could be effectively merged by making the two elements disjoint, e.g.

[{ files: ['**/*.js'], something: {a: 1} }, { files: ['**/*.js', '**/*.ts'], something: { b: 2 }]
=>
[{ files: ['**/*.js'], something: {a: 1, b: 2} }, { files: ['**/*.ts'], something: { b: 2 }]

Again, happy to put together some code to flesh this idea out.

[nzakas]

As with other suggestions, I'd be happy to evaluate a prototype.

Just as a reminder: we are potentially trading off additional complexity for performance, so I want to make sure the performance gain is substantial to offset that cost.