feat: add parse_outline_only support

[tiensonqin]

Related to #113

Benchmark result:

make bench 
dune exec ./bench/bench.exe
Estimated testing time 1m (6 benchmarks x 10s). Change using '-quota'.
┌──────────────────────────────────────┬──────────┬─────────────┬────────────┬────────────┬────────────┐
│ Name                                 │ Time/Run │     mWd/Run │   mjWd/Run │   Prom/Run │ Percentage │
├──────────────────────────────────────┼──────────┼─────────────┼────────────┼────────────┼────────────┤
│ Inline parse (doc)                   │   4.89ms │  1_846.67kw │   281.61kw │   133.81kw │      3.27% │
│ Inline parse outline (doc)           │   1.11ms │    454.27kw │    54.17kw │    54.17kw │      0.74% │
│ Mldoc Org mode parser                │ 149.58ms │ 84_303.34kw │ 1_857.82kw │ 1_857.82kw │    100.00% │
│ Mldoc Org mode parser (outline only) │ 123.64ms │ 68_350.74kw │ 1_620.65kw │ 1_620.65kw │     82.66% │
│ Mldoc Markdown parser                │   2.06ms │  1_067.95kw │    29.25kw │    29.25kw │      1.38% │
│ Mldoc Markdown parser (outline only) │   1.32ms │    915.82kw │    25.28kw │    25.28kw │      0.88% │
└──────────────────────────────────────┴──────────┴─────────────┴────────────┴────────────┴────────────┘
Benchmarks that take 1ns to 100ms can be estimated precisely. For more reliable 
estimates, redesign your benchmark to have a shorter execution time.

[tiensonqin]

@RCmerci I copied most of the tests from test_markdown to test_outline_markdown, also added some tests related to timestamps.