Optimize fbuffer_append_str_repeat

[byroot]

Helps with pretty printting performance:

== Encoding activitypub.json (52595 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     1.746k i/100ms
Calculating -------------------------------------
               after     17.481k (± 1.0%) i/s   (57.20 μs/i) -     89.046k in   5.094341s

Comparison:
              before:    16038.4 i/s
               after:    17481.1 i/s - 1.09x  faster

== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    60.000 i/100ms
Calculating -------------------------------------
               after    608.157 (± 2.3%) i/s    (1.64 ms/i) -      3.060k in   5.034238s

Comparison:
              before:      525.3 i/s
               after:      608.2 i/s - 1.16x  faster

== Encoding twitter.json (466906 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   160.000 i/100ms
Calculating -------------------------------------
               after      1.606k (± 0.5%) i/s  (622.70 μs/i) -      8.160k in   5.081406s

Comparison:
              before:     1410.3 i/s
               after:     1605.9 i/s - 1.14x  faster