parser/pageparser: Store shortcode names as unique.Handle[string] to save memory...

parser/pageparser: Store shortcode names as unique.Handle[string] to save memory allocations

```
                        │ stash.bench  │        perf-pagelexer.bench        │
                        │    sec/op    │    sec/op     vs base              │
ShortcodeLexer-10         68.87µ ± ∞ ¹   65.91µ ± ∞ ¹       ~ (p=0.200 n=4)
Parse-10                  7.421µ ± ∞ ¹   7.030µ ± ∞ ¹  -5.28% (p=0.029 n=4)
HasShortcode/Match-10     70.42n ± ∞ ¹   77.37n ± ∞ ¹  +9.86% (p=0.029 n=4)
HasShortcode/NoMatch-10   14.32n ± ∞ ¹   14.38n ± ∞ ¹       ~ (p=0.086 n=4)
geomean                   847.2n         847.3n        +0.01%
¹ need >= 6 samples for confidence interval at level 0.95

                        │  stash.bench  │         perf-pagelexer.bench          │
                        │     B/op      │     B/op       vs base                │
ShortcodeLexer-10         181.3Ki ± ∞ ¹   177.4Ki ± ∞ ¹  -2.19% (p=0.029 n=4)
Parse-10                  24.56Ki ± ∞ ¹   24.19Ki ± ∞ ¹  -1.53% (p=0.029 n=4)
HasShortcode/Match-10       0.000 ± ∞ ¹     0.000 ± ∞ ¹       ~ (p=1.000 n=4) ²
HasShortcode/NoMatch-10     0.000 ± ∞ ¹     0.000 ± ∞ ¹       ~ (p=1.000 n=4) ²
geomean                               ³                  -0.93%               ³
¹ need >= 6 samples for confidence interval at level 0.95
² all samples are equal
³ summaries must be >0 to compute geomean

                        │ stash.bench  │         perf-pagelexer.bench         │
                        │  allocs/op   │  allocs/op   vs base                 │
ShortcodeLexer-10         1004.0 ± ∞ ¹   921.0 ± ∞ ¹   -8.27% (p=0.029 n=4)
Parse-10                   34.00 ± ∞ ¹   14.00 ± ∞ ¹  -58.82% (p=0.029 n=4)
HasShortcode/Match-10      0.000 ± ∞ ¹   0.000 ± ∞ ¹        ~ (p=1.000 n=4) ²
HasShortcode/NoMatch-10    0.000 ± ∞ ¹   0.000 ± ∞ ¹        ~ (p=1.000 n=4) ²
```