string_buffer::write(double) truncates to 6 decimals with no way to raise it

[RedFox20]

rpp::string_buffer::write(double) keeps 6 decimals and never uses scientific notation, so a
value that needs more precision is written wrong and the caller gets no warning.

_tostring(char* buffer, double value, int maxDecimals = 6) in rpp/strview.h sets that
limit. The limit is not reachable from string_buffer, because write(double) takes no
precision parameter.

rpp::string_buffer sb;
sb.write(59.43696123456789);  // -> "59.436961"     6 decimals, the rest is gone
sb.write(1e-9);               // -> "0.000000"      the value is lost
sb.write(1e300);              // no scientific notation

This is easy to walk into. A GPS latitude written through string_buffer loses about 1 cm of
resolution per lost decimal, and the text still looks correct.

Suggested fix

Add an overload that passes the precision through, the same way _tostring already accepts
it:

string_buffer& write(double value, int maxDecimals);

A shortest round-trip form would be better still, so write(double) returns text that parses
back to the same value. std::to_chars(first, last, value) gives that.

Found while replacing a JSON serializer in a downstream project. We had to use Qt for the
double output, because string_buffer could not write the settings file correctly.