GPS: configure L76K for GPS, BeiDou and GLONASS

[F4FPR]

What

Configure serial L76K receivers for GPS + BeiDou + GLONASS during their location
provider start sequence, after the receiver reset step.

The implementation is guarded by the existing GPS_L76K board marker. It waits
250 ms after reset, sends the documented PCAS04,7 command through
MicroNMEA::sendSentence() (which generates the checksum and terminators), then
waits another 250 ms before normal processing continues.

Why

The ThinkNode M6 already identifies its receiver with GPS_L76K, but MeshCore
does not currently configure its constellation mode. The L76K defaults to GPS +
BeiDou; PCAS04 mode 7 also enables GLONASS and can provide additional satellite
margin.

This is not claimed as a proven fix for an intermittent 0 sats state. It is a
small, documented initialization improvement that can be tested independently.

Scope

• affects only builds defining GPS_L76K (currently ThinkNode M6, Heltec Tower
  V2, Nano G2 Ultra and ThinkNode M9);
• reuses the existing board marker instead of adding an unused opt-in macro;
• does not change UART baud rate, GPS power/reset handling or NMEA parsing;
• sends the configuration after reset(), because EnvironmentSensorManager
  calls begin() followed immediately by reset();
• does not send PCAS03, so GSA/GSV diagnostics remain available;
• does not send PCAS11, which is absent from the L76K V1.1 specification;
• adds 500 ms to each L76K start sequence.

The impact of enabling the additional constellation on receiver power consumption
has not yet been measured.

Source

Quectel L76K GNSS Protocol Specification V1.1, PCAS04 section. Mode 7 is
GPS + BeiDou + GLONASS. QZSS is enabled independently by the receiver.

Related to #2862 and #2863.