constructor() {
    this.buffer = Buffer.alloc(ONE_MEG);
    this.pos = 0;
    this.hasSegment = false;
  }
Efficient builder for base64 VLQ mappings strings.

This class uses a buffer that is preallocated with one megabyte and is
reallocated dynamically as needed, doubling its size.

Encoding never creates any complex value types (strings, objects), and only
writes character values to the buffer.

For details about source map terminology and specification, check
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit