charset: remove dependency on x/net for parsing html

[gabriel-vasile]

charset: remove dependency on x/net for parsing html
remove dependency on xml for parsing xml

pkg: github.com/gabriel-vasile/mimetype/internal/charset
            │  /tmp/prev   │              /tmp/curr              │
            │    sec/op    │   sec/op     vs base                │
FromHTML-4    1732.5n ± 1%   429.2n ± 0%  -75.23% (p=0.000 n=10)
FromXML-4      829.8n ± 0%   215.2n ± 1%  -74.07% (p=0.000 n=10)
FromPlain-4    192.8n ± 1%   192.8n ± 0%        ~ (p=0.670 n=10)
geomean        652.0n        261.1n       -59.95%

            │   /tmp/prev    │              /tmp/curr               │
            │      B/op      │    B/op     vs base                  │
FromHTML-4    4400.00 ± 0%     16.00 ± 0%  -99.64% (p=0.000 n=10)
FromXML-4      576.00 ± 0%     32.00 ± 0%  -94.44% (p=0.000 n=10)
FromPlain-4     0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
geomean                    ²               -94.13%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

            │  /tmp/prev   │              /tmp/curr               │
            │  allocs/op   │ allocs/op   vs base                  │
FromHTML-4    6.000 ± 0%     2.000 ± 0%  -66.67% (p=0.000 n=10)
FromXML-4     9.000 ± 0%     5.000 ± 0%  -44.44% (p=0.000 n=10)
FromPlain-4   0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
geomean                  ²               -43.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean