Implement expandGlob() and expandGlobSync()

fs/glob.ts:
- Improve prototypes for expandGlob() and expandGlobSync() from #604.
- Rename glob() to globToRegExp().
- Add normalizeGlob() and joinGlobs().
- Extract GlobToRegExpOptions from GlobOptions, remove the strict
  and filepath options.

fs/globrex.ts:
- Add GlobrexOptions.

fs/path/constants.ts:
- Add SEP_PATTERN.

fs/walk.ts:
- Add WalkOptions::includeFiles
- Default WalkOptions::includeDirs to true.
- Don't traverse directories matching a skip pattern.
- Remove walkSync()'s default root value.

prettier:
- Refactor to use expandGlob().

testing:
- Make findTestModules() an async generator.