feat(oxc_ast): add RegExpFlags bitflag for storing regex flags

[Boshen]

This reduces TokenValue from 56 to 40 bytes, Token from 72 to 56 bytes.

[github-actions]

Parser Benchmark Results

group                    main                                   pr
-----                    ----                                   --
parser/babylon.max.js    1.01    147.8±4.31ms    69.9 MB/sec    1.00    146.8±4.79ms    70.4 MB/sec
parser/d3.js             1.00     18.4±0.49ms    29.6 MB/sec    1.00     18.5±0.47ms    29.5 MB/sec
parser/lodash.js         1.00      5.2±0.28ms    98.0 MB/sec    1.00      5.3±0.19ms    97.8 MB/sec
parser/pdf.js            1.02     10.7±0.37ms    37.5 MB/sec    1.00     10.5±0.29ms    38.1 MB/sec
parser/typescript.js     1.00    146.2±2.90ms    65.8 MB/sec    1.00    146.4±3.58ms    65.7 MB/sec