Make pegs module compatible with javascript backend

[nigredo-tori]

It would be nice to have a regex-like library working consistently across targets. Pegs seems to be the best candidate, since it is pure.

Right now there are two problems I see:

1. transformFile uses readFile, which is missing in JS. Easily solved by wrapping the whole proc in when.
2. After that the module compiles, but fails for even the most basic cases. E.g. this throws an exception:

import pegs

assert "x".match(peg"x")

[timotheecour]

• snippet given works as of 9384f7a but nim js -r tests/stdlib/tpegs.nim fails
• nim js -r tests/stdlib/tpegs.nim works as of StringStream & more stdlib modules support for JS/NimScript #14095 but it needed to change the test for that to work
• if $(a: float) now works consistently in nim js, avoiding printing floats as ints #14134 is merged, the test doesn't need to change